Maddy Platter is Peggy's and Hoyt's ungrateful mother, Bobby's maternal and Luanne's paternal grandmother who resides on the Platter Family ranch in Montana raising cattle, voiced by Mimi Rogers. She was mean to Peggy as a child, and now holds a grudge against her for marrying her choice of a man (her husband Hank) instead of the son from the neighboring ranch. Doc Platter, her husband was implied to be deceased. However, he appeared in "A Rover Runs Through It". Although Peggy along with Hank and Bobby saved the ranch, Maddy was still harsh to her daughter.
- In earlier episodes, Maddy appeared as an older version of Peggy, with identical features. She was completely redesigned for "A Rover Runs Through It", as well as receiving a more abrasive personality. Later appearances would return her to her original "older Peggy" design however.
- In "A Rover Runs Through It", Peggy states that her mother ignored her all of her adult life. This is a complete contradiction to "I Remember Mono", where Maddy is clearly seen in the Hill's house and an argument between the two was the starting plot for "Happy Hank's Giving". However, given her harsh attitude in "A Rover Runs Through it" and not knowing or caring about Peggy's educational career, it was an accurate statement of Peggy to say her mother ignored her life in Texas.
- In "Square Peg", Maddy appeared in a flashback when Peggy was a little girl. Maddy looked very similar to adult Peggy and gave a her a book about flowers as she couldn't bear to have the sex talk with her. She seemed to be much nicer too
- In "Cheer Factor", Luanne Platter has a baby named Gracie, making Maddy a great-grandmother.