Something seems really odd about Marcia's mouth in this movie - like she's wearing dentures or something. They do some close-ups of her, and she has weird wrinkles around her mouth. Something is definitely up.
I think Christine Taylor had a lip augmentation by the second film. She did have very thin lips in the first, though she looked better natural. I don't see how it could be her aging as this film was only made a year later.