Let's face from the popularity of actors like Adam Sandler or Will Ferrell, to TV characters like Homer Simpson, Peter Griffin, Michael Scott, etc. the list of funniest movie or TV characters is dominated by complete idiots

What is it about being a dumbass that works so well? Does it make viewers feel better about their own intelligence? Is it just a good way to get into physical comedy situations?

I think the key is that the dumb person must think they’re smarter than everyone else.

It works when it's relatable. Everyone likes to think they're intelligent, but the truth is that most people are just normal and have made silly decisions before and have chasms in their knowledge ... so when a show highlights this with a dumb character, people relate but without ever drawing attention to themselves.