One of the questions concerning national costume that comes to mind for me when refering to an American national costume is the shear size of our nation. Not meaning to sound in any way condesending, but a lot of our states, especially those west of the Appalachian mountains, are larger than many countries. To combine all of the USA into one national costume would be like pulling all of Europe Union together and assigning a single national costume it.
Anywhere in the world that you venture the cowboy is easily recognized as being entirely of American origin, as would the general look of blue-jeans, t-shirt and baseball cap.