No dumb people are not necessarily the most confident but they do often appear confident. I think it’s because they are often too dumb to understand the subtle differences and facts regarding topics and issues. It’s probably not fair to call them dumb. It would be better to describe them as uneducated. They just don’t have enough knowledge to know that they’re often wrong.
They confuse opinions with facts and either can’t or won’t read or can’t really comprehend what they read. A good example of people in this category are President Donald J. Trump supporters and Trump himself.