I have posted fan fiction on the internet, starting over ten years ago, and I have to admit, I have toiled with the need to revamp and edit my older stories, badly, as some of the grammar isn’t consistent, and some of it applied to old grammatical rules that no longer apply.
However, you can’t necessarily say they are a bad writer. Look at Stephanie Meyer’s books, they got a bad review for grammar from Stephen King himself, and are highly popular books. So, I think the grammar doesn’t constitute whether the writer is good. A writer is, first and foremost, a story teller. Writing is an art form, as there are so many forms of writing, and types of writing that hiring an editor can make things easier, but it can also change the voice of the writer.
So, with due respect, as a writer myself, who has grown over the years, and has been a favorite of many readers in the past before I learned polished grammar, I say it’s the spirit of the writer that counts most.