The Amerimutt meme is mostly a joke. You foreigners view America through the lens of the New York/Los Angeles media. Hence your view of our demography is distorted.
Growing up in America, Hollywood/JewYork casting choices always seemed strange to me as a kid, for so many of the "whites" looked way more Jewy and Italian than the people I knew. Also, the media was always stuffing blacks and misc. ethnicities into everything, even though most white Americans live and work around other white Americans. (This is somewhat less true today.)
I spent most of my life in Nevada, Utah and Idaho. The heritage of my family is about 90% English, 5% Scotch-Irish and 5% Western European/Scandinavian. There must be more than a hundred million of us still in the USA with no non-European DNA.
Although I understand the critique of America's obsession with the multi-cult, with our being controlled by Jews (as if Europe isn't) and our creation of mountains of propaganda, the ethnic core of the USA remains white. You could carve a massive, single white ethnostate from the USA, so mocking us as if we're all "mutts" is ridiculous.
No doubt many Amerimutt threads are anti-white shills trying to divide us. They want white America to be considered a degraded, "mutt' people by other whites.