It looks like you're new here. If you want to get involved, click one of these buttons!
Fanworks that do not fit the tone of the original work sort of bug me (MLP ones being one example of this). I guess I mostly am thinking about super dark and edgy fan stuff of a work made for children or that is otherwise pretty lighthearted. I am not sure if there is really anything bad about the opposite type of original work/fan work tone dissonance and I have not encountered it that much anyway. Feel free to let me know if you have.
Anywho, I believe that children's cartoons, comics, games, and other works that were deliberately made to not have tons of violence, swearing, or sexual content offer a kind of refuge for people who dislike that sort of thing. I feel like the proliferation of fanworks that add the exact type of material excluded from those works intrudes on that little refuge.
Now, I agree that people should not just try to pretend that violence, cursing, and sexual stuff does not exist. That probably would be naive and unhealthy. There are tons of "adult" shows, books, and movies with that kind of content though and I do not really understand why it needs to be injected into works that purposely lack it.
I think a fan focus on darker and edger things can also make finding and talking with other fans of a series difficult. If you just want to discuss the last episode, book, or game for what it is and are not interested in sexualized fanart, some dark fanfic, or people's opinions of the attractiveness of certain female/male characters, can be pretty hard sometimes. Plus, in the case of media actually aimed at kids, I am not sure if it is such a good thing for web searches to turn up a lot of the aforementioned types of fanworks, but your mileage may vary on that.