In The Pigman, what is John's main problem?

I feel this question has to be answered by some reference to John's family situation, and in particular the way that his father is constantly pressurising him to join his profession and ignores John's own desires and ideas for what he what might want to do. Consider how his family situation is presented in Chapter Seven, and how his father brings up the topic of his working with him by suggesting that John goes and works with him after school. John's response indicates the profound lack of communication that occurs between himself and "Bore," as he touchingly refers to his father:

I almost choked on a mouthful of yams when he said that. I mean, I've bene over to the Exchange and seen all the screaming andbarking Bore has to do just to earn a few bucks, and if he thought I was going to have any part of that madhouse, he had another thought coming.

John's problem therefore in part stems from the pressure he is put under to follow in his father's footsteps but also at the same time his desire to follow his own path and explore the things that he is interested in. This leads him to be rebellious and to go against his parents as he seeks to assert his own will and discover his own likes and dislikes.

In The Pigman by Paul Zindel, what is the main characters' biggest problem?

By the end of Paul Zindel's novel The Pigman, the two protagonists, John Conlan and Lorraine Jensen, have realized that their greatest problem is their immaturity and that it is their immaturity making their lives what they are.

Both John and Lorraine feel alone and alienated by their families. John feels his parents refuse to understand him, making him feel bored with the world and act out by pulling pranks. Lorraine was abandoned by her father and rarely sees her hardworking mother. John's invented prank to telephone strangers leads the two characters to meet Angelo Pignati, who is equally lonely due to his wife's death.

The three develop a close friendship in which they all influence each other. John and Lorraine even influence Mr. Pignati into roller-skating inside of his house, strenuous exercise that gives Mr. Pignati a heart attack. While he's away at the hospital, due to their immaturity, John and Lorraine further take advantage of Mr. Pignati by throwing a party that results in many of his things being wrecked, including his wife's collection of porcelain pigs. Learning that John and Lorraine, whom he loved and trusted, actually cannot be trusted proves to be too much of a strain on him, and he dies of a second heart attack.

Sadly, had John and Lorraine behaved in a much more responsible, mature way, they would not have caused Mr. Pignati's death. Hence, they realize that it is really their own immaturity making their lives miserable, nothing else. As John phrases it at the end of the book, "Our life would be what we made of it--nothing more, nothing less."