I have been treated like people think I'm the village idiot, even though I score an above average IQ and was considered to be a so-called "gifted child" when I was a child, was academically bright (mostly an "A-student") and I'm not an idiot by most definitions of the word.
But because my processing can be slow, and because I don't instantly realize when sarcasm is being used, and because I don't "get" many NT jokes right away, and because instructions may be given in a manner that doesn't fit my learning style, I have been laughed at, made the butt of jokes, and consigned to the "she's not very bright" heap by numerous groups of people in various settings, from workplaces to social situations.
Not really in school because the other students were so aware of my brightness in the classroom, grades, and such, that nobody there thought of me as dim or stupid, because they knew I was academically near the top.
But out in the general world, where nobody knows about your "grades" in school anymore nor even cares, and they go by more socially based criteria, you are an idiot and forever after thought to be just for the crime of not picking up on a sarcastic joke.
I also don't get hints and "leading" questions or remarks, and I feel that this failure, too, contributes to someone who doesn't know me very well forming a conclusion that I'm a dim bulb.
Couple that with attempts at even being a little bit friendly and smiling because one is trying not to be a miserable grump, and they really think I'm dumb.