How do you treat people who you think are stupid?

It’s on my mind lately that it’s the general response of most people to mistreat those they think are stupid. That, in their minds, it’s deserved. I think that mentality especially shows up in customer service jobs, where customers think people working certain jobs are dumb.
But maybe I just think even stupid people should be treated with basic decency because I happen to be one of them. Of course I don’t want to be mistreated.