I'm especially curious about this in areas with a lower immigrant population.
In NYC, everyone and their mom flies flags associated with their family's backgrounds. Puerto Rican flags, Cuban flags, Jamaican flags, Polish flags, Indian flags… it's hardly uncommon. It's also common to have those little fringed flags you see in cars.
*Wearing* flags (like as clothes or on a keychain) is less common but not mind-blowing. I mainly associate it with latinos myself, especially in the summer and during parades.