As we all know, people can be REALLY dumb. And sure, its probably just your perspective of them that makes them so dumb. But in the situation of dumb people, you should always nod. Nodding and laughing a little and saying yeah is always a good thing when you are dealing with someone who is just dumb to you.
And yes, its mean to label someone like that, but we all do it. I try to be the nicest person I can be so it usually makes me feel really mean when I do that, but you need those simple tricks to deal with someone like that.