"Good country people" is a phrase that is echoed several times throughout this short story, and the connotation is that "country people" are honest, hardworking, and genuine souls who are in high demand and short supply. Mrs. Hopewell and Mrs. Freeman hold these "country people" in high regard, though the implication is that they also don't have much financially.
When Manley Pointer arrives, he quickly evaluates Mrs. Hopewell's character, and when she tries to send him away, he uses what he knows must be her own stereotypes against her:
"I'll tell you the truth – not many people want to buy one nowadays and besides, I know I'm real simple. I don't know how to say a thing but to say it. I'm just a country boy." He glanced up into her unfriendly face. "People like you don't like to fool with country people like me!"
This works. He both convinces Mrs. Hopewell of his "honest" character and gains entrance into her home. She replies,
"Why! ... good country people are the salt of the earth!"
Mrs. Hopewell is full of platitudes, and because this young stranger shares her same vapid sense of reasoning, she deems him harmless and in need of a good meal.
The irony is that this "good country person" she allows into her house has a bizarre collection, and he has marked Hulga's leg as a needed object that he wishes to acquire. By pretending to be a "good country person," Manley Pointer deceptively gains the trust of the women so that he can seduce Hulga for the sole intention of stealing her leg.
The irony is that there are no salt-of-the-earth, "good country people" in this story. Instead, the ironic title highlights the truth that all of characters are rather despicable and dishonest.