In my mind, parody is about exaggerating the show as it is or maybe common fanon of the show in order to make it funny.
Crack is all about taking some external, bizarre thing and inserting it into your universe so that the character reactions are funny. Like, turning everyone into girls! Or random dinosaurs! Or truth serums! Or more dinosaurs! Or robots! While the show/characters may be exaggerated to bring the funny, it's not to the same degree or with the same attitude.
I've seen parodies labeled crack, though, and just simple comedy labeled crack, and I think I've seen a few people asking what exactly the hell crack fic is supposed to mean. So yeah, maybe I'm just thinking about this all wrong. What do you guys think?