A quote I found here on HN, that I really liked: "Americans will say they are Italian because their great grandma ate spaghetti once, but God forbid someone is American because he was born there" - mvieira38 (https://news.ycombinator.com/item?id=43930642)
What I would ask you is why does it irk you, why do you care? Is it some hindance to my culture that I want to learn about it and try to "cosplay"? What would you prefer that we act as though we're here sui generis? Is somebody's culture lesser because they're not in that country at that time?
People of Italian ancestry in the US did not forget everything about their past, in many cultures that transition is even more recent; I remember my immigrant grandmother. Comes off as gatekeeping people who would otherwise be your relatives.
It doesn't keep me up at night, but I think it's tacky and vulgar.
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.