I am American and I have always been a fan of HM, PP and their children. I'm reserving admiration on the others.
Since earning a bloody freedom from British rule and the amount of time that has elapsed since that effort, there are a lot of folks who are just not interested. And that interest does not lie with just British royalty. Most people don't know the name of any European royalty, either.
However, I am a history buff, both stories about the people themselves and of their military histories.
I would venture that possibly, on a good day, 1% pf the American population would range from mildly interested to fanatic.
As for Wills and Kate, they are more celebrities than royalty.