for that matter Elizabeth Bowes-Lyon was, technically, a commoner. Didn't stop her becoming the nation's No 1 Gran ( I loved the horsey, gambling, gin-drinking Spitting Image puppet of her )
Diana was also a commoner, although a member of the aristocracy and descended from Charles II by way of various undisputed but illegitimate sons. Hence her title - Diana, Princess of Wales, whereas Princess Anne is so styled due to being royal in her own right, as is Prince Phillip
However the Duchess Fergiana was and remains, common as muck, whatever titles she acquires along the way.