Jonah Hill doesn't play any part in GTA 5, although you wouldn't be the first person to think he might have been Jimmy.
Why do people think Jonah Hill is Jimmy from GTA V?
I think it's pretty obvious that the reason people get mixed up is that some of the characteristics of Jimmy, such as the way he rants, are inspired by Jonah Hill though..
It'd be too much of a coincidence if they weren't!
The two also looks very similar.
By the way, the real voice behind Jimmy is Danny Tamberelli, who looks nothing like Jimmy from GTA V
Little fun fact though, Danny was a child actor that featured in the Disney movie Mighty Ducks!