It was an honest mistake, I don't understand how that makes him a moron.
See, I'd like to say that I avoid morons, but I interact with quite a few of them, and most of them are quite endearing. Casual oversights - especially ones that require pop culture knowledge and not, you know, actual critical thinking, don't actually factor into why they're simpletons.
Intelligence is, to take the definition, a hearty helping of abstract thought, understanding, self-awareness, communication, reasoning, learning, retaining and some other stuff, but that's all that's relevant to this example. The initial oversight doesn't render him an idiot, but had you had to explain for the next 30 minutes why "The Walking Dead" was a free to use term, and that the comic books were not the first or last to use it, and all he did was stare back at you like a slack-jawed yokel, then it'd be safe to say that you were dealing with a guenuine dipshit.