I see this thread a lot on my dash and I don't quite agree.
It's not anyone's job.
People's improvement doesn't depend on other people being gracious enough to throw (unsolicited or not) critique. No, people improve by doing whatever the hell they want with their art. https://twitter.com/rorschachisgay/status/1252608532769394688

If you paid for fanfiction, you would have your right to review but if you get this stuff for free, you magically lose that right unless someone asks for crit. Simple as that, no matter the content or intentions. Reader's power is in not giving attention to things they dislike.

In short: it ain't that serious, bro. It's fanfiction, people writing and exploring freely whatever their heart desires, from various backgrounds and of various skill levels. Not everybody want to be pro writers. Accept this.