As far as I know, no King or Queens has attended a US presidential inauguration. Hard to know if any have been invited. Those events are most important to fellow countrymen.
Diana brought the British royal family to center stage in the US.
Pre-Diana, most of us knew the bare minimum about the modern RF, including that the queen had a passel of kids and Charles was the heir. TBH, I couldn't have told you the names of any of her other children, didn't know if they were married, and didn't even know Philip's name, but was aware she was married.