* Zim fans (simple and straightforward)
* Irken fans (referring to the species of Zim and his allies)
* Zimsters (a playful and enthusiastic term)
* Zimbros (a less common, but more dedicated term)
You might also find them using the names of specific characters like Dib fans or Gaz fans.
Ultimately, the term used is less important than the shared love of the show!