"The Earthling figure who is Charles Darwin- who taught that those who die are meant to die, that corpses are improvements. So it goes." Darwin's idea of evolution teaches that people die for a reason. But in a sense, it seems odd that the Tralfamadorians would appreciate this idea since they wouldn't answer his simple question of "why?"
"Everything is beautiful and nothing hurt" Maybe the quote on his tombstone is a mark of his sufferings. All of the horrible times he faced during the time of war, yet somehow managed to survive. He is free of this world, he no longer exists in his previous reality which had been full of pain and suffering. It seems as if he had finally come to a place in which his sufferings could no longer get to him.