A good American is "an American who knows what is good for him". Or in other words, a USA citizen who knows that he is stupid, his government is smart, and his government knows what is good for him.
A good American pays all the taxes.
A good American doesn't care that secret millitary experiments are being held a mile from his house.
A good American supports the war (no matter where it is being held).
A good American buys American cars.
Danny: Hey Johnny, you wanna join our demonstration against the war in Iraq?
Johnny: No, I am a good American.