Have you heard the phrase "A Good Man is Hard to Find" before? At this point, it has become a proverb in American English. You might hear just about anyone from your grandmother to a six-year-old girl trying to be cute repeat this phrase. O'Connor herself didn't invent it. It's actually the title lyric of a song composed and performed by Eddie Green back during the time of piano rolls (1918). The song was a hit when it was first written, but became even more popular as a signature tune by Sophie Tucker.
O'Connor probably expected her readers to know the song, or at least the lyric that comes next:
A good man is hard to find
You always get the other kind
That certainly seems to be true in the story.
The title is also a line in the story; it comes into play when Red Sammy and the grandmother are talking about how no one is trustworthy anymore. They both agree that a good man is hard to find, particularly these days. The interesting question, though, is the following: what is a good man? Characters in the story certainly have something to say about that, but each one might offer a different perspective.