Did you even read Cicero's journals? He isn't a "dumb" character. My bet says that you didn't.
You probably didn't even do his starting quest before meeting the DB.
Who cares about the journals? I mean, if his acting is embarrassing and annoying and unrealistic, does the journal erase this from my memory? I mean, one of the problems I had with the Oblivion voice acting (particularly Dark Brotherhood characters) is that they felt like being from the "Cyrodiil theatre troupe", which is not a good thing. Cicero feels like being from the "Skyrim theatre troupe" in an even more annoying fashion. He's still a dumb character. Maybe he's somewhat intelligent or whatnot, I don't care if he's actually a genius, but Bethesda's writer were probably drunk when designing certain aspects of him (like when the voice actor was doing his work). And as you know, while it's fun drinking, doing stuff while drunk rarely gives good results.