What an idiotic straw man definition.
Of course America is a "a flawed nation state like many others, displaying a maddening mix of admirable and appalling characteristics."
The question is not one of whether American history, actions and values has been perfect and of unmixed goodness, it is one of the net effect and value of America as compared to the net effect and value of other societies/nations. While both are mixed, 70% good vs. 30% bad is significant when compared to 30% good vs 70% bad.
IMO, by that definition America is quite exceptional.
This "there is no black and white, only shades of gray" argument is a common one on the left, and is fallacious. While it is technically correct when dealing with any human activity, the purpose of making the argument is to imply, without saying so explicitly, that there is no real difference between dark gray and light gray, which is just stupid.
Well said.