Well, I really hated Dat offstage, but it wasn't really because he wasn't funny offstage. It was because he was delusional. And not just repetitive with jokes between shows onstage (as they all are), but repetitive between the jokes THEMSELVES, thematically.
A lot of decent comedians are "off" when not onstage. So they suck as TV-Glass-House-People but could indeed be the best comics onstage.