Is this thread a joke? The U.S. still is, these days, one of the most nationalistic countries on Earth! The whole "proud to be an American" thing, coupled with the absolute disinterest in foreign cultures is a trademark of the U.S. in this day and age, and back then, when the U.S. was still miles behind most of the civilized world in terms of human rights, that poster was quite suitable for the overblown, caricature context it's supposed to represent.
So, no. It's not offensive. If anything, it accurately portrays the POV of a vast portion of the American public, then and even somewhat now.