The message in the story is that you should not hold yourself above others. The message is delivered through the disagreeable Mrs. Turpin, who is racist, arrogant, and just plain rude.
The reason the message is delivered through a character that seems so disagreeable is that the message is one a disagreeable person needs to here. If an agreeable character got the same message, it would just be confirmation.
This woman needed to hear a message. The minute Mrs. Turpin enters the room, she expects people to make room for her.
There was one vacant chair and a place on the sofa occupied by a blond child in a dirty blue romper who should have been told to move over and make room for the lady.
Why does she assume that someone should make room for her? No one does, so she scoots her way in. She does not care if others are sicker and need the spot more than she does. Her grotesque fatness is a metaphor for her inflated ego.
However, Mrs. Turpin gets a wake-up call in the form of a college girl who throws first a book and then an insult.
The girl raised her head. Her gaze locked with Mrs. Turpin's. "Go back to hell where you came from, you old wart hog," she whispered.
Mrs. Turpin someone is able to get the message. She realizes that she has been rude. She is a horrible person, and not automatically going to get into heaven. This abysmal revelation is both disturbing and deep, but it would not have happened to a better person. If she had been a better person, she would not have needed a revelation.
The real question is whether or not Mrs. Turpin actually learned anything. This can be open to interpretation. On the one hand, she hears a chorus of cricket hallelujahs. This could mean she has come to her senses and decided to change her ways, or that she decides that she does not need to. It is in some ways up the reader to decide.