While Lou Ann is happy that her mother and Granny have come to visit (especially after her husband's abandoning her and the birth of her little boy), Granny especially gets on her nerves during the visit from Kentucky. Granny is a stereotypical stubborn, older woman who wants things her way. She doesn't like the climate and complains about that; she doesn't like the culture which differs from Kentucky's and complains about that, and she certainly does not approve of Lou Ann's "lifestyle."
In contrast, while Lou Ann's mother is not as negative as Granny, she wears on Lou Ann because Lou Ann is caught between her mom and her grandmother. She feels like a referee in their relationship and simply does not have the time or energy to play such a role.
While Kingsolver is adept at creating realistic characters, the dynamics among the three women are my personal favorite. I think that there are many humans who have faced similar situations like Lou Ann's and who must deal with too much "family time."