Here's most of the perspectives I've seen:
Some people work very hard to make sure all their fanart is technically legal under fair use. I'm not very familiar with the relevant laws, but I had friends who were -- they could tell you 'these prints count as parody, those prints change enough of the outfit to count as x% of the original design being changed and thus not being an infringement,' etc. etc.
Some people feel that selling fanart is okay because big companies can afford to take the hit. Some people feel it's okay because they are not hurting individual artists. Some people don't care if it's illegal- you can get away with it, and it's a lot harder to make money in Artist Alley if you don't.
It becomes more complicated, too, when you're dealing with anime and manga since in Japan, there's more of an expected culture of fanart, so you run into a weird thing where it's technically illegal in America, so the creators COULD crack down and order you to stop, but the Japanese creators of those works would be unlikely to see a problem with it.
Since it's not super risky, it's mostly an ethical decision. Personally, I decided not to sell fanart, but I just think that whatever choice you make, it should be a choice that you thought about, and if you decide to sell fanart, you need to consider who it is you're impacting every time you make a piece of fanart -- selling Undertale fanart hurts the creator a lot more than selling Pokemon fanart.
(As an aside, since I was surprised to learn this: unique commissions of copyrighted characters are actually fine! It's the legal equivalent of a band covering a song at a concert)
I REEAAALLY dislike a lot of what Disney's done to copyright laws, but I'll defend them on this -- my understanding is, they do this kind of thing because if they don't defend their copyright, it can be contested legally, and Disney would lose the ability to defend their copyright against the infringers they actually care about stopping. So they have to be pretty heartless about it because if they don't, they'll get taken advantage of.
It's really easy to say, as a smaller creator, "hey, I would be fine with fanart!" but fanwork can open up a lot of legal loopholes, and more visible creators and companies have to be careful about the precedents they set in order to protect themselves. :/