sure! before I say anything, I want to mention I only speak for myself. another poc might have a different opinion, and you should inform yourself based on a swath of others' perspectives. this is just mine!

"is it bad for a white person to write about poc?"
tldr: no.
personally, I would encourage everyone to create a diverse cast of characters whenever they can. if a white person takes care to research and be culturally sensitive, then there's no issue with white people writing non-white characters.

"can you elaborate how being white would change things when you write content about poc?"
tldr: race impacts everything, which means it also affects how/what a person creates.
white people writing non-white characters is not a problem by itself, and I don't believe I ever said that – my apologies if that's how my tweet came across. but a creator's race does affect how/what they create, because race affects everything: how you're perceived, how you're raised, how others treat you, etc. for this reason, race will always matter, even in content creation.

I've said it like 50 times atp but it wouldn't hurt to repeat: this doesn't mean anyone needs to disclose private information about themselves. everyone is entitled to privacy on the internet. regardless, a person's race still matters when it comes to creating characters, especially when the content is poc-centric.

even "good/proper representation" should not be absolved of potential criticism, because that metric is not objective. that criticism should take into account the creator's race.

this might seem like a bit much for "just fanfiction" but unfortunately, race is central to everything, and therefore matters, even in fanfiction and fanart. it doesn't have to inform whether or not you consume content from white or non-white creators. but it still matters.

I hope this helps and doesn't come off as preachy, I tried to get my point across as clearly as possible! sorry for the length as well. thank you for being polite and have a good week 🙇🏻‍♀️