Until the very end of the story, the narrator is staunchly anti-painter-lady. After all, she says at the beginning, "It was our wall, and she had no right coming into our neighborhood painting on it." The narrator is a child and loyal to the street they live on; the narrator does not yet see the nuances in people's lives and characters.
But even the narrator's mother loses her patience with the painter lady; while the latter is asking a lot of questions about the menu at her restaurant, "she was tapping her foot and heating up in a dangerous way." And when the painter lady asks for beets in her salad, the narrator's mother snaps a little:
"You will get," Mama said, leaning her face close to the painter lady’s, "whatever Lou back there tossed. Now sit down."
But later she regrets getting irritated at her and tells the narrator that the painter lady is from the North and isn't familiar with Southern politeness. She put herself in the painter lady's shoes, felt sympathy for her as a struggling artist, and said that she seemed "decent."
But, says the narrator, "Me and Lou definitely did not want to hear that." They resented their mother's concessions toward the painter lady; they didn't want to humanize her in their minds because then it would be difficult to dislike her. And they also resented their mother for being able to see things from the painter lady's point of view, because they were so hell-bent on ganging up against her.