Mary and Freddy aren't related.
Well that, and the fact that it was SUPPOSED to be disturbing. That was the point, to show how Mary, as an extension of the ever-increasingly potent Will of Darkseid, had been utterly defiled. She's soulless, without any direction, other than Darkseid; without any will, other than Darkseid's; without any purpose other than to do the degraded acts which make up her universe now, the universe of Darkseid.
That's one really interesting note that I picked up on re-reading it:
When Kara finally confronts Mary, and blasts her with heat vision, she says to her "There's still time to talk!" Now Kara must know, in her heart of hearts, that Mary is a lost cause, an arm of pure Evil. But Kara can't give up hope. She exhibits almost the embodiment of hope.