Contextual intent and artistic care are always what separate "fun and enjoyable fanart" from "bad attempted fix art".

If you say you're creating an alternate continuity for fun and use that as a starting point for some artistic creativity, alongside maintaining the basic likable qualities of the characters, you'll get positive interest, like the "Toasted-toro and Her Chocolate-Colored Friends" fanartist of Nagatoro.

Meanwhile, if you instead say "I fixed it" while pointlessly making the characters fat, ugly, devoid of the original personality, and (sometimes) a minority stereotype, you'll get very understandable major backlash.

Sorry if this is just saying what you said in more words, I just needed somewhere to mention my opinion about "creative fanart" versus "spiteful fix-art".

Both are cute and that's all that matters.