Cher

Let me start off by saying that I love Cher, and I love the idea of her singing Fernando (which was sadly lacking in the first movie).

BUT

I just rewatched the first one, and in one scene, Donna is talking to Rosie and Tanya, and, pointing up at the ceiling, says, "Someone up there has got it in for me. I'm guessing it's my mother." In a later scene, she says to Sophie something along the lines of, "When I got pregnant, my mother told me that if I left home, not to come back." So, from those two scenes, I have always inferred that, not only is Donna's mother not part of Sophie's life, but that she had also passed away by the time the movie takes place.

(And she's listed in the credits as Ruby Sheridan, so we know she's Donna's mother, not one of the dads' mothers.)