Erik and Arthur are laughing after Mike Costello's death because they are such insensitive, cruel boys that they find humor in Mike's little brother's reaction to the death.
Joey, Mike's little brother, quite understandably panicked and behaved strangely when his brother was suddenly killed. Joey became fixated on taking off Mike's shoes for him; in the process, he was getting in the way of the paramedics who were trying to resuscitate Mike. On page 51 of the book (in the chapter called "Tuesday, September 5") we find out about this, and we see then that Arthur and Erik are trying to keep from laughing and smiling as they describe Mike's death to Erik's mother and brother Paul.
A few minutes later, Paul witnesses Erik and Arthur in the backyard, laughing together and imitating Joey, saying "The shoes! Gimme the shoes!" They even laugh over how the lightning that struck Mike made his hair stand up: "He got Mohawked, man!"
Paul is appalled by their behavior, and he reasons that Erik and Arthur are actually happy about Mike's death because he had been the star player of the team, so now they will have a better chance of grabbing the spotlight themselves during football games.
Erik and Arthur's inappropriate and cruel reaction to Mike's death is an important instance of foreshadowing in the novel. That is, if Erik can find death genuinely funny, then there's something not right about him--he is at least bordering on being a psychopath. Readers will find out later other terrible things that Erik does. Mrs. Fisher's reaction is also an important instance of foreshadowing: if she notices at all that her son is laughing while informing her of his teammate's death, then she doesn't do anything about it. She's blind to the fact that her oldest son is capable of such cruelty.