You beat me to the punch, Setanta.
Out of fairness to Americans, I was just about to declare that people from my own country can be an embarrassment, too.
When I used to travel a lot more, I used to avoid Kuta (Bali) for that very reason. It was (probably still is) full of Australians behaving badly. Horrible.
But hey, to be fair to everyone, I don't think bad behavior in public is necessarily the characteristic of the citizens of any
particular country, really .... it's just that you cringe a whole lot more when they are from your own.
The most embarrassing moment I can recall, off the top of my head: standing in a ticket que at a railway station in Amsterdam quite a few years ago, I struck up a conversation with the woman who was standing behind me. Who turned out to be an America & who turned out to be very LOUD. After declaring that Europe was a real let-down, not half as interesting as she'd expected & that she couldn't wait to go home ..... she declared extra loudly, "This city is so DIRTY!"
It was kind of mortifying to be associated with her in any way.