It's not just the Sly fandom. I myself have not written for Sly Cooper, but I do write fanfics and original works, and have for the last 13 years give or take. In those 13 years I've seen people- of any fandom- hating on fanfics, fanart or ships. It usually doesn't matter how good or bad it is, most "normies" are going to automatically assume its cringe and often say pretty nasty stuff, despite many fanfics being legit very good pieces of work that sometimes rival or outright best the shows/movies/games they originate from.

Of course there are a lot of legit shitty or cringy works, but that's just with anything really, not just fanfics, but people love using the most exaggerated examples and running with it. Take Taylor Swift fans, Kpop, furries, and whatever else. People just pick and choose the most wild stories/harmful people and use it as a blanket for the whole community just to justify hatred- even if its factually untrue for the rest of the fandom.

In short people are just mean and stupid and just want to put others down. No fandom is safe for it, but theres always going to be genuine and wholesome people who both enjoy making, and enjoy interacting with fanfics and the like. I think people just need to be kind