Respectfully disagree. He's certainly an exaggerated character, but so is everybody else in Boondocks. And, while he's usually best in supporting roles, The Story of Jimmy Rebel is one of the funniest things I've ever seen; as far as I'm concerned it's proof he can pull off the leading-man thing.
The Color Ruckus was a bit more uneven; I don't think we needed Ruckus's origin story, but on the other hand meeting his abusive father goes a long way toward explaining why he is the way he is -- while still managing to make his father a sympathetic character, tragic, bitter, and broken for his own very legitimate reasons. It's not my favorite episode by any means, but it could be a roadmap for how Ruckus can work in a family setting -- his brothers acting as the straight men, regular guys who feel bad for him and are embarrassed by him, but hey, he's still their brother.
Ultimately I don't see why an "Uncle Ruckus with new supporting cast" movie has to function on any different dynamic than "Uncle Ruckus with the Freemans" or "Uncle Ruckus with the Duboises", except that in this case he's the main character. (Hm -- come to think of it, Tom Dubois is another character who could be trivially translated to live action with the same actor. Maybe the easiest of the entire cast.) That definitely sounds like something you don't want to watch if you don't like Ruckus in the first place, but I like Ruckus and I think it could work.
