The title points to the theme and style of Flannery O'Connor's story. O'Connor is known for her realism that veers into symbolism, and this story is no exception.
The grandmother is the type of Southern woman who believes in social decorum and being a "lady," even as her family falls down the social spectrum. Saying a "good man is hard to find" is a way to convey one's own higher status against a world of deficient characters. Others do not meet one's refined sensibilities, both in status and in morality. Good thus has a double meaning, and despite her pretension the grandmother falls short on both measures.
The title comes into play when the family stops to eat and the grandmother speaks to Red Sammy:
"People are certainly not nice like they used to be," said the grandmother.
"Two fellers come in here last week," Red Sammy said, "driving a Chrysler. It was a old beat-up car but it was a good one and these boys looked all right to me. Said they worked at the mill and you know I let them fellers charge the gas they bought? Now why did I do that?"
"Because you're a good man!" the grandmother said at once.
She reiterates the claim with The Misfit, who again she assesses based on appearance:
"Listen," the grandmother almost screamed, "I know you're a good man. You don't look a bit like you have common blood. I know you must come from nice people!"
"Yes mam," he said, "finest people in the world." When he smiled he showed a row of strong white teeth. "God never made a finer woman than my mother and my daddy's heart was pure gold," he said. The boy with the red sweat shirt had come around behind them and was standing with his gun at his hip. The Misfit squatted down on the ground. "Watch them children, Bobby Lee," he said. "You know they make me nervous." He looked at the six of them huddled together in front of him and he seemed to be embarrassed as if he couldn't think of anything to say. "Ain't a cloud in the sky," he remarked, looking up at it. "Don't see no sun but don't see no cloud neither."
"Yes, it's a beautiful day," said the grandmother. "Listen," she said, "you shouldn't call yourself The Misfit because I know you're a good man at heart. I can just look at you and tell."
This becomes the running motif between The Misfit and the grandmother up until he shoots her, with the assessment that
"She would of been a good woman," The Misfit said, "if it had been somebody there to shoot her every minute of her life."
The horrific encounter does shake the grandmother out of her shallow sensibilities and moves her to a moment of genuine introspection and compassion, however brief it is.
Three types of goodness (class, behavior, and inner morality) all combine to support the theme of genuine human virtue in a complex world.
The Grandmother believes that "good people" are the ones who have "good character". She bases her opinion of the Misfit on the fact that he doesn't look "common", so he must be a "good" man. Her belief that a "good man" is hard to find comes from her own self-righteousness, the belief that she's better than others, her judgmental views, and her selfishness. She lives in the past and thinks it's much harder to find "good" people of "good" character than it was in her time. It isn't until the end that she realizes part of the reason for the Misfit's behavior is because of people like the Grandmother. The title is related to all of the themes of God and religion, violence and cruelty, the lack of a connection among people, and prejudice.
The story's title points directly to the main theme. O'Connor believed that "a good man is hard to find," and therefore we are all in need of grace from God and forgiveness. In the story, all of the characters are terrible, selfish, and prejudice human beings. The only character who shows some positive qualties is the Misfit, and he, of course, is also a serial murderer. O'Connor uses this exposition to establish the theme of human depravity and the need for grace for all of us.