I think this is a great question. Before analyzing this topic, it is important to understand the reason that Flannery O'Connor wrote this story and all of her other stories.
O'Connor's main purpose in this story, as well as her other writings was to bring people to Christ.
So, grandmother was absolutely a misfit, in the since that her actions were very un-Chistlike. She was hypocritical, prejudice, selfish and self rightous.
And the Misfit was obviously a misfit because he was a murderer, very un-UnChristlike indeed.
In a sense, they are equally misfits at one point in the story. They are equally sinful. I don't think that you could say that say that one is more of a misfit than the other.
However, at the end, the grandmother, before she is shot, is redeemed. She sees the error in her ways. She reallizes that the Misfit, whom she looked down upon, could have very well been her son. She didn't literally think that he was her son, but this shows how she no longer thinks of her self as better than others. It is indeed her moment of redemption before her death.
Certainly, it appears that the Misfit, unlike the grandmother, has few delusions about himself:
I call myself The Misfit...because I can't make what all I done wrong fit what all I gone through in punishment.
Nor does he have delusions about life:
Does it seem right to you, lady, that one is punsihed a heap and another ain't punished at all?
The Misfit understands the depravity of man, but the grandmother and her platitude about "a good man is hard to find" believes herself righteous. She constantly preaches to her family--although they ignore her. Her judgments are poor, from her decision that it is better to bring along her cat than to leave it alone at home, or her fatally wrong idea to go past a plantation house that is in an entirely different state. Of course, her final act of deluded thinking is that the three men will not harm her and her family. It is only before she dies that the grandmother has her moment of truth:
Why, you're one of my babies. You're one of my own children!
She finally realizes that she,too, is a sinner. Her sould is depraved just as the soul of The Misfit is depraved. Only in death is she thus redeemed:
'She would have been a good woman,' The Misfit said, 'if it had been somebody there to shoot her every minute of her life.