Dill feels horrible about what they did to Tom so he wants to shut away all of the horrible things that people do to one another and instead laugh.
Dill says this after the trial. He feels helpless and angry about Tom's conviction. Now, he says that he might as well just ignore the horrible way that people think about each other and treat each other, because he feels that he cannot do anything about it.
The character understands that he cannot change the world around him and their misconceptions. Instead of fighting with those people he is going to live his life and be happy.