This is actually a question of ignorance, because the main diagnostic criteria in autism is that the child or adult clearly shows advanced intellectual skills in one area or another, possibly more than one area. People who don’t know what autism is will ask this question, and isn’t that they are “dumb” ignorant, it’s just that they have no knowledge in this area and assume that people with autism are mentally retarded. Autistic children and adults face their challenges, but mentally retarded they usually are not.
Autistic people happen to be very smart, almost and often gifted, in the areas they excel in. People without autism are astounded at how seemingly extremely intelligent people with autism are. Then when they encounter the difficult subjects and areas that challenge the people with autism, it almost takes them aback. It’s as if two minds exist in one body; the super intelligent and gifted, and the slow and incomprehensive.
Still, it’s safe to say that people with autism are most definitely intelligent and smart. They show it in very different ways than the rest of us, but they are smart. They also have very different ways in approaching things, and “thinking outside the box” is obvious for them.