Mary: The Queen of Nag
I remember watching reruns when I was younger and always got a chuckle out of it here and there. Well, when I began watching it again as an adult, I realized what a drag Mary was. She nagged everyone to death. She butted in to everyone's business and always thought she had the right answer.
If you look at the series as a whole, probably two-thirds of the episodes involved Mary running her mouth and making a mess of the given situation. Poor Brenda for having a mother who never seemed to want to allow her to be a normal teenager.
Don't get me wrong, I think it's great that the show tried to celebrate a strong, sassy character, but she really ended up coming across as a bitch more often than not.
Sandra was the best part of the show and the main reason I would often watch an episode. She was so fun and, as someone else pointed out, Mary was so obviously jealous of her.