I met a guy who said “in the UK it’s only old, white, and racist people who display the flag”. It’s the same in the United States, but old, white, and racist people are more common! Half joke aside, it’s common in the US, but the typical caveat that the US is a large and diverse place applies. It’s common in some parts and not in others. I suspect there’s a high correlation in flag flying and affiliation with a certain US political party.