closest to an "all-American" costume is Western wear, derived from original American pioneer clothing, traditional in Texas, the Southwest, and many rural communities in the Western US; includes cowboy hats, Western shirts, boots, jeans, prairie skirts, and bolo ties.Various forms of Native American clothing. Traditional Southern wear includes white seersucker suits and string ties for men, and sun hats and large dresses for women.
The United States doesn’t really have a national costume the way that the Japanese have kimono or whatever. There also isn’t a royal/imperial past involved, so American culture doesn’t tend to focus on pomp and circumstance the way that a lot of European countries do when they’re celebrating themselves.
Insofar as I look at it, there are basically two.
Baseball caps. Jeans. T-shirts. These aren’t ever worn for ceremonial reasons, though. They’re just clothing items that are associated with Americana. They’re also worn by people all over the globe.
Suit. If you look at the equal of American pomp, the participants are basically dressed the way they’d be dressed for any other business situation. Americans don’t typically dress to impress. They dress for biz-nasty.
