I find it odd that she and Boethiah along are in the same category of good as Azura and Meridia who are considered genuinely good. Boethiah wants you to sacrifice by decieving a friend to her alter, and Mephala is similar but by her ebony blade but the only difference being she could be considered a gossip.There are other Daedra who have more honor than those 2 yet they are part of the holy trinity and yet Meridia is discarded? Meridia is more on par with Azura. Mephala and Boethiah are more comparable to Dagon and Baall with their indulgence of cruelty and deciet by the impressions they've given me playing Skyrim.
Like Azura, Mephala and Boethiah helped the Chimer and the Prophet Veloth. Mephala taught the Chimer to use stealth and deception against their enemies and Boethiah was the one to show them to Morrowind and also open them to new ideas of worship. So when it comes down to it both are really only good to the Dunmer.
Boethiah is the Daedric Prince of unlawful overthrow of authority, and is therefore seen is presiding over rebellion. He is also considered to be the Dunmer's god-ancestor, and therefore they worship him occasionally, certainly more than other races.
Mephala is thought to be the precursor to Vivec by the Dunmer, so they replaced their worship of her with Vivec. The Dark Brotherhood was formed to honor Mephala, and some believe that the Night Mother is really Mephala in another form, since she is the Daedric Prince of assassination. Therefore she could be seen as good, since the Dark Brotherhood has been used to kill off some of the great tyrants of history.
I think it makes a lot of sense when you consider that Boethiah is the opposite force to Molag Bal. Alongside Mehrunes Dagon, Molag Bal is almost universally seen as the Daedroth most closely resembling evil by everyone in the mortal realm except his followers and vampires. Boethiah hates authority, especially tyrants like Molag Bal the god of domination and rape, over all else and has the sphere of unlawful rebellion against authority. It is true that Boethiah supports the overthrow of all authorities both good and bad, but for sure Boethiah's agenda weakens tyrants as a group, and after a successful rebellion Boethiah is not going to like a newly installed tyrant any more than the one that was overthrown; in the real world, rejection of tyranny eventually gave rise to democracy, so there is definitely an argument to be had that over time a similar effect could occur in the Elder Scrolls universe if Boethiah becomes influential enough. Boethiah's agenda also weakens Molag Bal's agenda, since the more successful Boethiah is the fewer and fewer tyrants are going to exist in the world, and the more successful Molag Bal is the more Boethiah comes across to mortals as a helpful bulwark against Bal's evil. In the long term though, it seems like Boethiah is only going to be satisfied with a universe without any real authority whatsoever, which is interesting to think about when considering what that would mean for both Nirn and the Aurbis as a whole.
Edit: To clarify, attributing "evil" to Molag Bal is entirely subjective on the part of mortals; but since Molag Bal is almost universally reviled amongst mortals, Boethiah being Molag Bal's rival makes Boethiah seem more attractive by comparison. Plus there's the view that Boethiah helped the Chimer and Dunmer a whole bunch so of course they like Boethiah.
Arguments for Mephala's "goodness" on the other hand are less straightforward but no less valid. Even though she is a goddess of murder, when one considers how many people one murders even when doing "the right thing" in the Elder Scrolls games, her good qualities are more apparent. Consider also her agents, the Morag Tong, who are notably more restrained and honorable than their rivals, the Dark Brotherhood. The Morag Tong believe in controlled killing that prevents greater evil and loss of life in the long run; this is very similar to the Assassins of the Assassin's Creed franchise, who I'm sure you'll agree are on the whole more "good" than not.
Mephala's also the Daedric Prince of sex, and sex is usually seen as a positive thing by most people. And she's also the Daedric Prince of secrets, and if you believe that privacy is a good thing, then you have to acknowledge that secrets definitely also have their place.
Ultimately though, the "goodness" or "badness" of Daedra comes down to the values of the person judging them. An ethically responsible prostitute could certainly be a life-long devotee of Mephala, and a revolutionary could likewise see Boethiah in a positive way and for good reasons. In the end, all the Daedra are amoral entities who act upon their natures, and whether or not we see them as good or bad says more about us and what we value in the world and in ourselves than it does about them.
It's been a while since I added anything to this thread, but to add something to the discussion of Boethiah's "goodness", I just wanted to make a contrast between Boethiah and Mehrunes Dagon, who are both gods who encourage destruction of authority but, while Boethiah is seen in a more mixed light by the people of Tamriel, Mehrunes Dagon is often hated more than Molag Bal. Why is this?
Mehrunes Dagon is the Daedric Prince of destruction, revolution, energy, change, and ambition (and some also say hope, too, or at least something approximating it). The key word there is 'revolution', and another important word is 'ambition'. Boethiah is the Daedric Prince of sedition and rebellion: the unlawful overthrow of authority, but not revolution per se.
So what is the difference between rebellion and revolution? In the former, people are primarily trying throw off the shackles of another individual or collective, maybe so that they can continue to survive and/or become free to do their own thing, which often though not always involves destroying the individual or collective in question. But revolution is more ambitious than that. (See me referring to "ambition" again?) Revolution seeks not just to overthrow a regime, but to replace it: one authority becomes replaced by another authority reflective of the revolutionaries' hopes and dreams rather than those of the previous elites'. So as to make ones dream a reality, one imposes it onto other people by forcing them into submission. In doing so, the revolutionaries become the new elites, and the members of society who are not part of the ruling class become the new underclasses. As long as elites, old or new, are amiable to him and his goals, Molag Bal is all about tyranny and domination, but Mehrunes Dagon isn't satisfied with just replacing tyrants; Dagon must eventually answer the call of the newly oppressed and seek to satisfy their hopes of empowerment. People like Mankar Camoran tease their followers that Mehurnes will lead them to a new dawn where they shall be alike to the Magne-Ge and know true freedom, but in truth there is only an endlessly repeating cycle of ambition and destruction: a hollow promise of hope in which both the weak and the strong are expunged ad infinitum.
In contrast, Boethiah does not believe in the hollow mercy of revolution, only the struggle of rebellion. An individualist, he or she sees mortal existence as a constant fight for survival and self-actualization. In the eyes of Boethiah, even trust can be a liability that keeps one bound to chains. Hope is something that must be earned, as it cannot be otherwise, and the only way to earn true hope is by obtaining the radical freedom to push past all limits, internal and external. Notions of sin and objective morality are both obstacles to overcome. This is why Boethiah comes across as so confusing to mortals; how can the same entity heroically battle monsters one moment only to later strike down someone who pleads for mercy? It is because while Boethiah cares for mortal life as a whole, he or she is constantly pushing mortals to better themselves: "tough love", as it were. Boethiah wants to push the weak to become strong so that they have the power they need to decide their own fate, and accordingly only blesses those who demonstrate a willingness to push past their limits, sacrificing some to encourage others to become better (as an example of Boethiah's philosophy in action, see the story 'Boethiah's Proving'). Laziness in self-improvement is not tolerated by Boethiah (see Skyrim's Boethiah questline), and Boethiah's champions are made to understand that Boethiah will seek to betray or challenge them again in the future to further the pursuit of strength for themselves and their successors. According to Vivec, Boethiah is a big fan of Lorkhan, and he or she is determined to see the experiment of Mundus succeed.
This is why Boethiah is "good": because Boethiah wants mortals to improve and to transcend even him or her self. Interestingly, according to 'Elder Scrolls II: Daggerfall', Boethiah also has a curious relationship with the followers of Stendarr. This seems paradoxical since Boethiah isn't usually about mercy at all, but maybe while their methods differ the two gods regard one another with mutual respect. Sounds like the set-up to a good story, in my opinion.