There is a definite difference between the John at the start of the book and the John at the end. Note how the first chapter gives us an accurate idea of John's character through the way that he sets himself up against the establishment of school. He sets off firecrackers in his school and organises other students to roll rotten apples towards substitute teachers. He drinks and smokes, and definitely is presented as an anti-establishment figure. As the novel progresses, we see that this is largely in response to his relationship with his parents. John deliberately tries to inject fun and rebellion into his own character in response to the way that his parents lead such a conventional and boring existence. This explains his happy-go-lucky nature and the way that he throws a party in Mr. Pignati's house. It is often he that has to drag the reluctant Lorraine with him along on their plans to cause mischief.
However, at the end of the novel, after the disaster of the party and the way that he recognises he was at fault, it is clear that John has learnt a massive lesson. Note what he says towards the end of the final chapter:
There was no one else to blame anymore. No Bores or Old Ladies or Nortons, or Assassins waiting at the bridge. And there was no place to hide--no place across any river for a boatman to take us.
John is forced to recognise that, with Mr. Pignati's death, there was something in him and Lorraine that died as well, and that now, they had to face the consequences of their actions. John realises that he cannot "blame" anybody else for his life, and that his life would be "what we made of it--nothing more, nothing less." The symbol of the monkeys is one that is tremendously important, as the last sentence of the novel indicates:
They build their own cages, we could almost hear the Pigman whisper, as he took his children with him.
John learns that he must not "build his own cage" of irresponsibility. He has to accept responsibility for his own life and the decisions and mistakes that he made just as he has recognised that he was wrong to throw the party. He has moved from being a carefree, rebellious youth to a position of maturity.