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 婊子, 子 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