Although this is work filled with humor, prejudice is probably the main problem in Peck's novel.
The following excerpt from the analysis at eNotes should help explain the issue for you. Follow the link below for more information on social sensitivity and other issues Peck addresses.
"The townspeople are prejudiced towards the homeless men who ride rails seeking work. Prejudice is the result of misunderstanding, lack of education about other people and their ways, or emanates from a mean spirit and selfishness. If readers identify areas of prejudice in themselves, they can find several instances where Joey and Mary Alice learned from Grandma Dowdel how to treat others with compassion. She came to the defense of Shotgun Cheatham's reputation, Mrs. Wilcox's dignity, Aunt Puss Chapman's material needs, helped rescue seventeen-year-old Vandalia from her abusive mother, and fed hungry people. When we do not know the circumstances surrounding a person's life, we might prejudge that person. Peck's characters show a better way to treat others."