I totally agree with what everyone else has said. Cosplay whoever you want, and if people complain, they're either jealous cuz you're sexier than them or they think they rule the world, which they don't. ^^
We totally have an entire Invader Zim group for Kumoricon 2008, including Zim, Dib, Tallest Red and Purple, Gir, Gaz (maybe two of them X3 ), and possibly Prof. Membrane, as well as Emo Zim and Dib. It'll be a giant non-anime party! XD