I’m sorry to tell you but fanart is subjected to copyright, as are all derivative works that aren’t sufficiently transformative, even if they aren’t used commercially. It’s a subjective measure but I doubt any judge would say those top images are completely distinct from the Minions or Simpsons. What happens is that usually the rights owners don’t chase every single infringement, out of goodwill or simply because it would be too expensive to litigate every unauthorized use.

To be fair personally I think that’s excessive. But I believe so especially because it makes artists lives more difficult. However AI isn’t making it any easier either…