I have a feeling sedusa was abused as a child. Her behavior when she was disguised as Ima Goodlady certainly wasn't just to keep the girls out of the picture, like when she LITERALLY STOLE OCTI FROM BUBBLES. Now, It's impossible to know where this pettiness comes from, but since she was acting as a parent, a past of abuse sounds like a VERY good guess.
While one's upbringing can play a role in a child's life to determine whether or not a child becomes an abuser as an adult, it's not the only determining factor. Plenty of abuse victims grow up to not be abusive. Plenty of abusers grew up in happy, healthy homes.
That being said, Sedusa's treatment of the girls in "Mommy Fearest" isn't at all indicative of how she was raised as a child. She's only being mean because she's a villain, and the Powerpuff Girls are her enemies. Of course she'll use whatever leverage she has on them and use it to her advantage.