Ugly Americans are those American tourists who expect the locals to behave like themselves.
these tourists do not know to modify their behavior when they visit a different culture.
not only Americans can be "ugly" but they have the worst reputation.
see also ethnocentric

The origin of this term is somwhat ironic. It comes from Eugene Burdick and William J. Lederer Vietnam-era novel, a critique of American policy in Asia. The title character of the novel is not the stereotypical insensitive American clod; in fact he is the one who understands the culture of Asia accurately. He is called the Ugly American due to his unlovely appearance.

Often known as "goddamn gaijin" (goddamn foreigner) in Japan.
In Asia they can be spotted violating dozens of social customs at any given time, laughing at the locals because they are different, and complaining about whatever pops into their ugly little heads.

Living in a foriegn country is usually a good way to correct the habits of ugly americanisms, but only if you are willing to learn something.