But she's not an American. She never was.
She was created by an American writer, but that writer did not try to make her into an American. He tried to make her into a propaganda poster for what he believed America should be: a matriarchal society built on BDSM play. That's not what America is supposed to be.
Over the years, she became a great character with her own ideals and values to represent. Those values are largely in line with American values, but they are not uniquely American concepts. As long as Diana stays true to those values, she'll always be what she's always been. What she wears doesn't matter.