It would be like you encountering someone on the street, and they have bags of groceries in their hands, and you ask them, “Oh, did you go to the grocery store today?”
And they stare at you blankly, and ask you what in the fuck a grocery store is.
That sounds…stupid, but I’ve read stories with that sort of shit in them. It happens a whole lot in fantasy novels, when authors are trying to get world exposition out of the way, so they literally have grown characters explaining world geography to one another like they’re both fucking idiots, I hate that shit, it’s a waste of life.
That’s pretty banal though. Worse is when the cluelessness is tied into the development of the actual character or the conflict of the story, which happens more rarely. There’s also the obnoxious version, which is the “hyper-meta-aware” character played for jokes–see “Deadpool”, and the too-smart-for-their-own-good teens in “Cabin in the Woods”, where one overly clever character rules all of the other characters clueless by comparison.
Bracken's Corner 