disfiguring acne. Neither make any attempt to adhere to the beauty myth of their culture by wearing makeup, pretty clothes or styling their hair. Furthermore, neither makes any attempt to be pleasant and charming, or smooth over rough patches in social situations, which are chief attributes of a "lady." Hulga even deliberately chooses the ugliest name she can think of to replace her birth name of Joy, which to her signifies all the falseness and hypocrisy of the society around her.

We only perceive Mary Grace through the limited and narrow-minded viewpoint of Mrs. Turpin, who judges the young woman by completely conventional norms and feels smugly superior to her. She blames Mary Grace for glaring at her, deciding the young woman is unpleasant and rude, rather than perhaps wondering if something in her own behavior might be motivating the hate vibes. We are, in contrast, much more privy to Hulga's interiority—like Mrs. Turpin, Hulga makes the mistake of feeling superior to the people around her, most notably Manley.

Mary Grace brings truth to Mrs. Turpin when she throws a book at her and calls her a wart hog. In contrast, Hulga receives truth from Manley, learning that her arrogance about her knowledge and sophistication have been misplaced as she encounters true evil.

In many ways, Hulga is the Mrs. Turpin character in "Revelation," receiving grace in a surprising way. In contrast, though she does not seem to be evil, Mary Grace fulfills Manley's role in "Good Country People" of bringing grace to another.

The character Mary Grace from the short story "Revelation" and the character Hulga from the short story "Good Country People" share many traits. They are both young white, women residing in the American south who are described as "stout" with bright blue eyes. Beyond physical similarities, they share some personality traits such as being highly educated. Mary Grace is an undergraduate student at Wellesley College and Hulga holds a PhD in philosophy. In addition, they are surly and unfriendly to those around them, in particular, their mothers and their mothers' friends. In all of these ways, the two characters very closely resemble one another. In addition, both characters are physically helpless at the end of each story: Mary Grace sedated on a stretcher and Hulga stuck in the loft of a barn.

The main difference between them is that Hulga's personality develops to include a vulnerable, hopeful side whereas Mary Grace does not receive a description past the grumpy, rude side of her behavior. In fact, Mary Grace is sedated by a doctor because she throws a book at another character's head. Hulga is not physically violent, but rather is the victim of the traveling salesman who tricks her and steals her wooden leg, leaving her marooned in the loft of a barn. Both characters are outsiders in their community for not displaying the stereotypical polite, friendly charm of a young southern woman and being depicted as differently abled mentally or physically. However, Mary Grace victimizes someone else by using violence while Hulga is a victim of violence.

While both Flannery O'Connor's stories have the theme of “redemption through catastrophe,” Hulga and Mary Grace differ in certain ways, although they share many similarities.

Differences

• Mary is the agent of the redemption in "Revelation," while Hulga is the recipient of the redeeming revelation in "Good Country People."
• Hulga believes in nothing; Mary Grace studies Human Development.
• Mary Grace recognizes the evil in Mrs. Turpin, 

"Go back to hell where you came from, you old wart hog," she whispered.

However, Hulga falls victim to Manley Pointer's evil. 

Similarities

• Mary Grace's name is certainly indicative of her role in Mrs. Turpin's receiving of the grace that can lead to her redemption after her dream that reveals her sinfulness. In a similar significance of name, Hulga's having changed hers from Joy indicates her acceptance of the ugly nothingness of life.
•  Just as Mary Grace recognizes the evil in Mrs. Turpin, calling her "old wart hog," like Hulga she enjoys causing Mrs. Turpin pain.
• Both Mary Grace and Hulga are disfigured physically
• Both Mary Grace and Hulga are disgusted by the banal conversations of the people in the waiting room and Mrs. Freeman and her mother. They both refuse to engage in conversation with the objects of their scorn.
• Both girls hate hypocrisy. When Mrs. Turpin begins to feel sorry for Mary Grace, the girl looks at her "as if she had some special reason for disliking her." Likewise, Hulga slams the door to her room when her mother talks with Mrs. Freeman she scowls, showing her disapproval of them.
• Mary Grace is classified with "freaks and lunatics"; Hulga, too, is a "freak" as she has the artificial leg.

There is some lack of clarity in your question. There is no character named "Mary Grace" in "Good Country People," though Hulga, of course, is critical to the story. She is educated, angry, superior, self-sufficient and subject to living on the periphery of life, somewhat due to self-ostracism. Do you mean Mary Grace from O'Connor's other short story, "Revelation"? If so, perhaps you would repost your question with more clarity as to the particulars.

Flannery O’Connor’s Roman Catholic beliefs color her stories which include violence and violent people alongside the faith and grace found by many of her characters. “Good Country People” and “Revelation” have at their core grotesque figures who struggle in a harsh world.  Often, in her stories, O’Connor uses a grotesque character alongside of one who is far more outrageous.

Two characters---Jo-Hulga Hopewell in “Good Country People” and Mary Grace in “Revelation”---are two of the outlandish characters that O’Connor uses to demonstrate to the world the importance of grace and redemption.  In their respective stories, Joy and Mary Grace have some similarities but also contrast in their ages and roles in the stories.   

Joy, an older woman, begins the story as a miserable, self-conscious woman who has a Ph.D. When she was a child, Joy literally had her leg shot off.  She claims to be an atheist. 

Joy’s counterpart is a Bible salesman who places her in an unusual situation: he wants to make love to her.  To prove her love for him, Manley Pointer asks her to take off her prosthetic leg and show it to him. Pointer steals her leg and her glasses and leaves her up in the barn loft.

It is then that Joy-Hulga must question her atheistic stance.  She will never be the same.  The author writes:

“…the girl was left, sitting on the straw in the dusty sunlight.  When she turned her churning faced toward the opening, she saw his blue figure struggling successfully over the green speckled lake.”

Joy-Hulga's  moment of grace  occurs as a result of Pointer's betrayal of her.  Things will never be the same for Joy-Hulga. One way or another, she is affected by the man’s cynicism and her openness to hurt.

Mary Grace in “Revelation” also has a self-concept problem.  A fat, ugly girl with acne who is about eighteen or nineteen—Mary Grace attends Wellesley College.  The girl is obviously unhappy and wears a permanent frown.  She and her mother have a contentious relationship which makes the girl a walking -time bomb.  Her counterpart enters the doctor’s office in the form of Mrs. Turpin, a large, middle-aged lady, who is a judgmental person.

Through her banter, Mrs. Turpin immediately becomes the focus of Mary Grace’s scowls and faces.  Mrs. Turpin tries to enter into a conversation with the miserable girl with an unusual result: the girl throws her book and hits the lady on the head; then, Mary Grace begins to try to choke her.  The nurse and doctor sedate the girl, but not before her eyes roll back in her head.  This indicates that the girl may be epileptic which might explain the odd behavior prior to the seizure.

The older lady asks the girl what she has to say to her:

The girl raised her head. Her gaze locked with Mrs. Turpin’s. “Go back to hell where you come from you old wart hog.”

It is not Mary Grace who has the moment of grace in this story but rather she is the one who sets Mrs. Turpin on the path to her epiphany.  Mary Grace’s  derogatory statement to the older lady made Mrs. Turpin spent time thinking about her racist remarks first with hurt, then anger, and finally with understanding.  She even has a vision of heaven.