While the story is written using the third person point of view, O'Connor uses the grandmother as the principal character and tells the story mainly from her perspective.
When analyzing the point of view in "A Good Man Is Hard to Find" we can look at the fact that certain details are provided that are only known to the grandmother and certain details are not provided that are impossible for her to be aware of.
For instance, the reader is made aware that the grandmother has brought her cat along for the trip and is keeping it hidden in a basket. The other characters are unaware of this fact until the basket is jostled and the cat escapes, causing a car accident. When her son is taken into the woods and shot, the grandmother cannot see him. Instead, she only hears the gun shots.
"There were two more pistol reports and the grandmother raised her head like a parched old turkey hen crying for water and called, 'Bailey Boy, Bailey Boy!' as if her heart would break."
This calculated presentation of details that align with the grandmother's perceptions/experience creates the accurate impression that the story is being told from the grandmother's point of view.
Presenting the facts and details of the story from the grandmother's point of view is consistent with the thematic and character interests of the story as the grandmother's tendency to judge (harshly) is confronted in the Misfit and his violent judgement of Christian morality.
If the grandmother faces a challenge to become a true Christian in the end, we have to see her moral point of view as a principal element of the narrative.
The conflict in the story is somewhat complex but relates essentially to ideas of moral systems as they are embodied by individuals. The grandmother's sense of rightness and righteousness causes a series of events (the family is lost on the road and the secreted cat causes a car accident). When the family is finally discovered by the Misfit and his group, the grandmother's role in this disaster is clear. Her decisions, notably, all derived from a staunch sense of her own justifications and the correctness of her own judgement (over and above that of her son and family).
When her moral sensibility is shown to have practical flaws (to put it mildly), the grandmother is left to face a man with an even greater sense of his own rightness. Thus the grandmother's perspective is central to the thematic interests of the story as they relate to moral perspective, righteousness, etc.