In Slaughterhouse-Five, the primary upshot of what Billy Pilgrim learns from the plunger-shaped aliens is: if we cannot change anything about time, there is no such thing as free will. After all, free will means the ability to alter your own future. In fact, the Tralfamadorians tell Billy that the whole idea of free will seems to be unique to Earthlings. Everyone else in the universe knows better. Billy uses this knowledge to comfort himself about the realities of aging, death, and pain. Even if human beings have to suffer, at least there is nothing to be done about it.
We don't think Billy's resignation is necessarily a good thing. Sure, it makes him feel better. But it also lets him off the hook: if you can't improve the world, why bother? The book comments that it is in moments of struggle that otherwise one-dimensional figures become real characters. One example is Edgar Derby's confrontation with Howard W. Campbell, Jr., when the narrator tells us that, "There are almost no characters in this story . . . But old Derby was a character now" (8.4.1). What makes Derby a character is his willingness to try to do something he thinks is right – and yeah, he may not achieve anything, but at least he tries.
Edgar Derby only becomes a character when he chooses to stand up against American Nazi Howard W. Campbell, Jr. It's this decision to stand up for what he believes in that distinguishes Derby from other people in the novel.
When Billy chooses to tell the world about Tralfamadore, it's the first and last truly independent decision he makes. However, his effort to make his own choices gets undercut by his daughter and the general public, who all think Billy is crazy. Everyone in the novel operates under so many social and familial constraints on their freedom that the attempt to make one's own choices appears insane.