It's possible for people to perceive you as less intelligent than you are for many reasons. Physical clumsiness, stimming, talking too little, talking too clumsily, talking in a certain way...
Important distinction: Your mom said "People think you are dumb". Strictly, she didn't say that you ARE dumb; only that people perceive you that way. She may or may not be right. Of course, what people think hasn't got much to do with what you actually are, especially since autism makes it harder for them to read you, same as it makes it harder for you to read them.
I don't know if it was rude for your mom to say that people think you're dumb because you don't talk much. Maybe she was trying to get you to talk more. Maybe she was just annoyed that you don't talk more. Maybe she's being an overbearing parent and trying to change how you act "for your own good" (because moms in many cases try to control their kids so the kids act the way the mom wants)...
The actual statement itself does not literally include an accusation of low intelligence. It is very much like the statement, "If you keep chewing with your mouth open, people will think I didn't teach you good manners!"