I wouldn't consider it inherently impolite; it depends on the tone of voice. Usually, it just acknowledges a difference in opinion or taste, which isn't impolite. Some people may think it's impolite, but usually not.

Yes, I think so. Most of the time that I've seen "suit yourself" used is in impolite situations. Often, my mother uses it when my sister or I want our way. When others become irritated with you in a discussion, they might say "suit yourself." Or the person may be tired when they say "suit yourself." I seldom use "suit yourself" because it just doesn't have a very friendly sound to it. Sometimes though, people use 'suit yourself' when their personalities might be a bit more rough or aggressive to begin with. People might excuse them for that because they understand the person is not being rude but that is just how that person is. But this is a quite rare occurrence.