Trivia: Maddy starts to film her movie and Mr. Mosby tells her that he is ready. Look at what he does right before she starts filming. He chants "Mah, mah, mah" and does the same thing Ashley Tisdale does in "High School Musical" before performing.
Trivia: London Tipton was supposed to be somewhat named after Paris Hilton. London and Paris (both cities) and Tipton and Hilton (Hotels).
Answer: Keep in mind, Muriel wasn't very popular. So the writers/directors must have thought it'd be a good idea to let go of the character. No one seems to be complaining.