I've noticed this also, but there's somehting else in Zim I haven't been able to place. His speech is very remensciant to much of Kevin's dialogue in various sketches, most notable the one's where he's insane; but there's somewhere else that comes from and I can't remember where it is.
I always figured it came from a cross between Kevin and the brilliance that is our voice actor Mr. Horvitz and that something else I just can't place.
BTW Just read on yourmom that MTV Networks is taking Zim to Cannes to perhaps put it up for sale, which could be hella GREAT news if it's true. Go Zim!