Who played mom on the John mother-in-law was by whom?

The mother-in-law in the sitcom "The John Larroquette Show" was played by Katherine Helmond.

It seems you might have gotten the title of the show slightly mixed up. It wasn't "The John Mother-in-law Show," but rather "The John Larroquette Show." The show followed John Larroquette as a successful defense attorney who suddenly finds himself divorced, living with his mother-in-law, and having to deal with his teenage daughter.