I don't mind nicknames in fan fictions that much, when they are used sparingly, but quite often I read a fanfic and I don't have a clue who anyone is because they always and I mean always use nicknames and it can be really confusing:
Har's P.O.V.
Hermy, Ro and me walked past Prof. Gona and Prof. Sna talking. We bumped into Dray, Goy and Crabby. Then we went to the common room and sat by the fire with Free, Gee, Gi and Ille. Then I dreamed of Vo-Vo and Trixie dancing then Lulu, Dob and Siri killed them.
I know none of the fanfics are as bad as that (I hope) but the point is people use extremely confusing nicknames and they are confusing. Please dear fan fiction authors use characters proper names and if you do want a nickname please use them sparingly.