Well, 'oh hell no' is an expression of strong disapproval or refusal. In the context of 'Harry Potter fanfic', it might mean that someone really doesn't like a particular Harry Potter fan - created fictional work. Maybe they find the plot too outlandish, or it goes against their idea of the original Harry Potter universe.
The characters need to be relatable again. If they've been distorted, go back to their original traits and build from there. Add more depth to them by giving them proper motives and emotions. Also, consider the overall tone. If it's too dark, adding some lighter moments or a glimmer of hope can improve it a great deal. It doesn't have to be all doom and gloom. And finally, get some feedback from other fans or even non - fans who can give an objective view on what needs to change.
Well, it could also be that the creation process of this fan fiction has been a hellish experience for the writer. Maybe they faced a lot of difficulties like writer's block, negative feedback from other fans, or problems with staying true to the original source material while trying to be creative, and all these issues have manifested in the final product as a 'hell - like' fan fiction.
Someone might say it if the fanfic contains really bad writing. Like if the grammar and spelling are atrocious, it can be a turn - off. Also, if the story is just plain boring or the characters are not well - developed.