fanfics make you... dumber?

No, I think some of those are written in very high qualities. I speaks Cantonese, and English is just my second language, so reading fanfics actually improves my vocabs and grammars a lot .. lol I wrote one fanfics about Slayer's Phibrizzo Hellmaster long long ago, perhaps it's one of the stories which might make people dumb.

No, but they make you punch the screen when you read them :) Seriously, if you are not skilled with english then you can learn bad habits from the fanfiction writers.