the vast majority of fanworks are bad, and that's fine, actually. they are bad for the same reason that the average number of legs for a human person to have is less than two: statistics. like with all endeavours and especially creative ones, most people who write fanfiction or draw art of their favourite characters are bad at it. if you line up all the crochet projects in the world, most of them will be, well, bad. some are bad because they're the first thing a person ever made, or the second or third or tenth, and this kind of thing takes practice. others are bad because the person who made them is just not very good at it. maybe they just learned how to make granny squares and they're perfectly happy to never expand or improve on that. most people who dance or bake or garden or braid hair are not amazing at it! and you'd never go to your kid's dance recital or eat your friend's homemade carrot cake and expect the same experience as you'd have at a professional ballet performance or award-winning bakery. And that's if we assume there is an objective measure of Good Art, which there isn't! Some art is just "bad" because you don't like it!
I think though that specifically with fanfiction, we sometimes forget that when we read a book or watch a movie, dozens of people have looked at it and given feedback and made changes and done quality control before the final product reaches our shelves or screens, and that's not counting the original writer's learning process and past experience. A published book is not anyone's first crochet project, even if it is their debut novel. But with fanfiction, the barrier to entry is so low (on purpose! this is a good thing!) that we do get to see a lot of wonky granny squares, and on sites like AO3 they're sitting on the same shelf as the hand-made silk lace wedding dress and you can't always tell just by looking at it which is which. The consequence of this is that we encounter fic that we think is unpolished, has bad punctuation, is out of character, and we are tempted to think "well, this is awful! how dare this person put this wonky granny square on the same shelf as the lace wedding dress!" But that's not how fandom is supposed to work! That wonky granny square is somebody who is really excited about this TV show they just watched and they are reaching out into the void to share their excitement with you. To scoff at them for not making a lace wedding dress is really, really rude. Even if they did make a lace wedding dress, maybe it's just really not your style, or you think they should have used a different pattern, and it's still their wedding dress. You don't have to wear the dress and you don't have to read the fic.
We all know that there is some fanfic out there that is incredible. I think it's important to talk about that! But the vast majority of people who post their writing online are just sharing their little hobby projects that they make for fun and I also think it's important to remember that.
actually I want to share a fun story about this
a couple of years ago I spent nine months working on a 22k AU fic. I put so much thought and care and attention to detail into it. I crafted every aspect of the AU around the canonical traits and abilities of the characters. I poured my heart and soul into this fic.
about a month ago, I got a notification that someone had posted a work inspired by that one. it took the basic premise and transplanted it into a fandom with none of the original details — someone had just read my fic, gotten excited about it, and swapped in their preferred blorbos just because. and it was delightful.
if that had happened a few years earlier, I might have been upset by it. how dare someone steal this idea I worked so hard on, and ignore all the care I put into it? but now, I just think it's great. I spent nine months building this gorgeous, elaborate dollhouse, and somebody liked it so much they wanted to play barbies in it. and now they get to have fun with it too! and so do all the people who read their fic! and that's awesome!
anyway tl;dr: as soon as you internalize the "holy shit two cakes" mentality you will have so much more fun in your fandoms
also let us not forget that a lot of books and movies that have gone through many levels of editing. are also bad.