I don’t think Kate WAS a commoner. In the books, her dad was an officer stationed in India. Officers at that time were gentry (but usu not titled. So an earl’s brother, not the earl). I know they changed it a bit on the show, but he was high enough in the diplomatic ranks to rub elbows with rajahs. And Mary was the diamond of her season, so she was ton. Also, performers were considered lower than working class. Women in the field were frequently mistresses or otherwise “for hire.”