I do believe that Superman is an American character. I believe this because he represents the best of what it means to be an American, and has many characteristics unique to the American experience that people who are from the US pride themselves on. But even more than the literal interpretation of his character, Superman is highly symbolic of the American experience in much more than just his back story. While some may argue that Superman is not an American character because of the ethnic nature of his creators and the issues of dealing with race in the subtext of his story, I believe that it is these very facts that do make him an American Character.
The story of Superman is the ultimate idealizing of the American experience. An immigrant boy is adopted by two "all-American" parents, from whom he learns wholesome middle class values. He is raised in the Heartland, and so is exposed to the toil of farm life and the values associated with earning an honest living from the soil. He goes to the city to seek his fortune, and proves that he is incorruptible in the face of urban degradation. He is simultaneously a perfectly assimilated specimen and an individual who remains deeply in touch with his cultural roots. He fights the bad guys. He has the power to make a difference in a world where it seems like the ideas and actions of individual matter less and less. He saves the day. He gets the girl. He is everything people dream of being. He is a superman.
Superman speaks to many of the dreams and fears of the greater American consciousness. While over the years he has changed to address the finer distinctions of contemporary concerns, the heart of the issue remains constant: Americans are unbendable in the face of adversities and remain true to the ideals on which the country was built. Now, while this mightn't be true for most, or even many, of the people in today's society, it is reassuring to have it represented for our imaginations to enjoy. Superman in an everyman's hero, someone that everyone can relate to (via Clark Kent) because there are always times when we feel like we're being swept along with the rest of society. But he is also someone that everyone can aspire to be like, not in the sense of having super powers, but in the sense of having the power to defend our beliefs.
Superman is not only an American character; he is the American character. Even if he was not originally, he has grown to be a practical embodiment of American ideology and values. He represents a common experience and is a part of a key aspect of American identity, immigration. Superman, whether in a cape or glasses, is, and likely will long remain, a symbol of national pride and identity in the American mind.