Well as an outside observer I think of a bunch of relatively smart people locked in a passionate but ultimately futile debate with each other based almost purely on the political party they support.

It would be hard to picture the average american because you do have so many cultures no one picture would encompass the whole. I think that the idea of eating fast food and dressing up as the American flag and having a firearm is probably how other countries would ignorantly portray an american. I do not really agree with the carefree anime depiction, If I had to draw someone I would draw a worn out/stressed out wage slave who is dedicated to the American dream.