The title phrase is repeated about half a dozen times in the story, and the idea of "good country people" drives much of the action. As we discuss in the "Tone" section, O'Connor doesn't seem to think much of Mrs. Hopewell, who thinks she gets to decide which people are "trash" and which are "good country people." We gather, however, that to belong to either category a person must be poor, white, and have little or no education.
Whether a person is "trash" or "good country people" (3) depends on the judgment of wealthier, more educated people, like Mrs. Hopewell and Hulga, though there is no indication that Hulga herself thinks of any people as "trash." While both women look down on good country people condescendingly, however, they also both trust them. And this is important.
Mrs. Freeman invades Mrs. Hopewell's kitchen at every meal because she probably knows this irritates Mrs. Hopewell. It's her way of exerting control and power over the person she depends on for everything, and whom she knows looks down on her. But Mrs. Hopewell can't imagine that Mrs. Freeman is deliberately antagonizing her; she can't imagine that she would think to or dare, so the constant mealtime lurking (and quiet frustration on Mrs. Hopewell's part) continues.
Similarly, Hulga agrees to go with Manley because she thinks he's good country people and therefore incapable of taking advantage of her or harming her. In fact, she fantasizes about tricking him into having sex with her, against what she thinks are his Christian values, and helping him reach "a deeper understanding of life." She thinks she can blow his little bumpkin mind. But in the end, she totally misjudges Manley, and the only person whose mind gets blown is hers.
The title, then—"Good Country People"—sets readers up to notice the moments when these words are uttered and, in doing so, to question their validity. It's a subtle shout-out to the upheaval that comes in the climax, to the notion that the "good country people" maybe aren't quite so "good" as they seem.