We’ll look at it this way. When elderly patients are rude, you won’t take it personally because you know they are unwell. Look at it this way that your customers might also be unwell, physically or mentally, and that’s why they are rude.

Also I think you might be assigning motives to people. Like they think you are dumb because you are a cashier. Maybe because you yourself think this job is dumb and kind of beneath you you think others might feel this way about you. But they likely can’t care less. When I shop or eat out the last thing I on my mind is that these people must be dumb. Why would anyone think that? Maybe if you try not to look down on your own job title/task, you’d not think others do either. Be proud of what you do. It’s a hard job

There are many reasons people work in retail: They like it, they think it’s fun, want to grow into management role, going to school, need extra income or just for a paycheck etc Dumbness has nothing to do with it.