Most common last names by state:
https://blogs.ancestry.com/cm/whats-the-most-popular-surname-in-your-state/
Interestingly no German surnames show up in the top three in any state, not even in Wisconsin. Besides the obvious Spanish dominance in California, Texas and the Southwest, and Asians in Hawaii - you see a few ethnic identifiers (Sullivan in Massachusetts, Olson in North Dakota, more Johnsons and Andersons in the Upper Midwest).
This isn't the best way to measure ethnicity, though. Some ethnic groups have a wider distribution of surnames than others. In particular, Spanish surnames are much more concentrated than English surnames, so while 1/50 people of primarily English ancestry might have the surname Smith, closer to 1/20 people of primarily Latin American ancestry might have the surname Rodriguez or Hernandez.
