Why? Other than looking superficially more fun-loving and relaxed than the monumentally fucked-up Charles, is there actually any evidence that she was a particularly inspired parent? She herself also came from a very dysfunctional background, was brought up by servants even before her mother left and her father got custody, was shipped off to boarding school, hated her stepmother, so she had no good parenting models or experience of what normal family life looked like.
Lots of royal biographers refer to her using the very young William as her emotional crutch, crying and ranting about Charles's infidelity and her own unhappiness, and treating him like an adult confidante. She arguably weaponised the press to win public sympathy at the expense of her children's wellbeing. It can't have been easy for her sons to be subjected to the Panorama interview, with all the revelations about infidelity, bulimia, suicide attempts, and the way she tipped off the press for 'candid' photo ops that showed her being a fun mother to the boys to win her war against the palace. Plus her own affairs with Will Carling and co. And subjecting them to a media circus on that holiday on the Al Fayed yacht.
I mean, with the best will in the world, she was a volatile, unstable, manipulative individual with some very poor judgement, not surprisingly, given her dreadful childhood, unhappy marriage and life in the public eye. None of that translates into Mother of the Year.