Which would have been wrong on just about every level.you could think of.
Firstly because the Queen part of Queen Mother comes from the title of the actual mother, who will then also be Queen Dowager (Queen Mary was Queen Mother twice) not the fact it’s a daughter who is Queen. I guess because the longest serving Dowager Queen who was known as Queen Mother did have a daughter as Queen, the meaning became blurred (Also when the Queen came to throne Queen Mary was still alive and Queen Dowager, so calling her mother Queen Mother differentiated between the Old Queen and the potentially 2 Queen Elizabeths. I think you have to go back a long way to find another time when there were 3 overlapping Queens)
So, if Diana was to be known as anything it would be Queen Mother, because King Mother as a designation doesn’t actually exist … which she couldn’t be because she would not actually have been Queen.
She would just have remained Diana, Princess of Wales unless she remarried or unless William reinstated the HRH and/or she was given a title in her own right.