It could be for a more personal reason. Maybe he's going through some internal struggle or self - doubt, and wearing a mask is a way for him to create a new persona, a stronger and more confident version of himself. It's like a shield that allows him to face challenges he might not be able to as the regular Naruto.
Perhaps the mask is just a fashion choice or it might be related to a specific story or event in the BT21 storyline that gives a reason for Mang to wear it.
Well, it could be that Kakashi, being a character who values the integrity of the original Naruto narrative, sees fanfiction as a deviation. Fanfiction might include out - of - character scenarios for him or the other characters. He might also dislike the fact that some fanfictions can be of very poor quality in terms of writing and plot, which could be a reason for his dislike.
Maybe he thinks fanfic often distorts the real story or characters of Naruto. In the original Naruto series, Kakashi has his own understanding and relationship with Naruto. Fanfic might create some scenarios that go against his view of Naruto or the world they live in.