Who's the biggest simp in cartoon (or anime) history?

I am not sure actually. None really comes to mind to even remotely challenge Mordecai for this label.

Does PaRappa the Rapper count, because that's honestly the only one I can think of XD
I'm gonna change everything about my life to get with some sunflower bitch who really only likes me when I'm gonna shit myself. That's the PaRappa way XD

Raku Ichijou from Nisekoi. He's pretending to go out with a hot blonde, due to family reasons, but actually likes a nerdy girl and is massively invested in trying to get with her.

(He's also the son of a mob boss xD)

Note: The bottom 3 in this order: Chitoge, Raku, Kosaki