RE4 isn't a zombie game? :P

Nope.  The enemies look kinda like zombies, but they can communicate with each other and use weapons, and do all sorts of stuff that zombies can't.  And the plot explains how they're different.

Zombies Ate My Neighbors is basically already Chibi.

So, with that in mind, either Dead Rising (which doesn't take itself seriously, anyway), or Resident Evil (a series which doesn't mind making a playable character out of a piece of Tofu).

Chibi Onechanbara: Bikini Zombie Slayers would be the best of course.

Frankly, any developer that makes a chibi zombie game is the winner.

And High Voltage is coming close, with their zoo zombie game.

Chibi Dead Rising... thousands of them on-screen would be awesome :D