It irks me because it usually manifests as embracing cartoonish stereotypes of the most superficial aspects of the culture: "I'm 1/64th Italian, so I like pizza. I'm 1/16th German, so I like beer. etc."

It doesn't keep me up at night, but I think it's tacky and vulgar.

It might usually manfiest as that or you're picking out the most superficial parts of people's identity to criticize. It's just not how I and others view it when we think about where the people who made us come from.

Or, to put it another way: your criticism is tacky and vulgar. Perhaps what you're describing is "cosplaying" but that's not how immigrant communities see themselves. I do in fact know the perecentages of my national makeup but pizza and beer aren't how I celebrate that. Nobles know their ancestry down to the smallest detail, is somebody really tacky for knowing that technically they are 1/4th Italian? I don't think attacking somebody's identity is ever fair; it costs you nothing but is everything to them.