When Mayella begins testifying, Jem supposes that she is using fake tears to win the judge's sympathy. Jem is wise enough to understand that Mayella is wrong to accuse Tom of any crime. But he is also smart enough to understand that her father is probably the one who beat her up and that she is just a pawn in her father's game of putting the blame on Tom. So, Jem thinks Mayella is certainly guilty of lying but he may have some sympathy for her as Scout does.
Jem thinks Bob Ewell is an ignorant drunk. There are no redeeming qualities about him. When Atticus embarrasses Bob in court, Jem also thinks of Bob as a threat. Atticus calms him down, telling him that Bob got his anger out of his system by spitting on him at the post office. However, Jem is correct because Bob continues to hold the grudge and attacks the children near the end of the novel.
Jem is upset that Tom is convicted. He says that he used to think the people of Maycomb were the best in the world. Miss Maudie tries to give him some hope and says that people like Atticus and Heck Tate did try to help Tom. At this point, Jem should see that Tate is fair-minded and one of the good guys.
Jem believes Tom is innocent. Jem knows how deceitful and ignorant Bob Ewell is, so he is certain that Bob is simply using Tom to take the fall for him. Other than Tom's family and community, no one is more upset at Tom's conviction than Jem. Jem doesn't come right out and say it, but perhaps he realizes that Tom is a "mockingbird," someone who does nothing but offer goodness to the world.