Susie's Mother is the mother of Susie Derkins. She was seen in one strip from the waist down. She also was referenced by Susie in another strip.
Susie's Mom

Calvin meets Susie's mother.

Her mother also appears off-panel when Susie goes to Calvin's house to ask for eggs and she asks if they gave her eggs, but Susie, meeting Stupendous Man, said no one was home. In yet another strip, she called Calvin's Mom because Calvin dropped a bowling-ball-sized snowball on Susie's head from the top of a tree as Stupendous Man. When he was caught, he quickly said that he was selling the snowballs, commenting later that his "Plan B"s leave a lot to be desired.