It really depends how it is done. most pseudo fans don't bother me.

People who enter a fandom and pretend to be fans of something, just so they can post things simply for the sake of creating wank irritate me. People who comment on fanart/fanfiction who aren't familiar with what the story/show/anything else the fanart/fanfiction is written/drawn for tend to just make themself look really stupid. (For example, well after HBP came out, I had someone leave a comment on a piece of fanart I had drawn of Blaise Zabini to tell me I got it wrong and Blaise is a girl)