Did the actress that played mother on waltons quit the-show?

You're likely thinking of Mary Jackson, who played Olivia Walton, the mother on The Waltons.

While she did not quit the show, she did leave after the 1981 season. This was because the show's producers decided to shift the focus to John-Boy's family, and Olivia's role had become more limited.

It wasn't a decision made by Mary Jackson, but rather a creative choice by the show's producers.