Although she's a minor character in the story, Joe's mother serves two important purposes.
For starters, she is the motivation for Missie May to change her mind about leaving Joe. She runs into her mother-in-law and decides,
Never would she admit defeat to that woman who prayed for it nightly. (103)
Can't let mommy win, right?
The second purpose she has is to inform Joe that the baby Missie gives birth to is indeed his. And, woohoo, it's a boy! Joe's mother makes only a couple appearances, but each time they're pretty significant in moving the plot forward.