In Slaughterhouse-Five, the primary upshot of what Billy Pilgrim learns from the plunger-shaped aliens is this: if we cannot change anything about time, there is no such thing as free will.
We suppose there are worse lessons to learn from aliens (or toilet plungers).
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.
But 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?
Questions About Fate and Free Will
What connections does the novel seem to draw between having "character" and having free will? Who are the real characters in the novel, if any?
Why is the Tralfamadorian idea of time incompatible with free will?
Does Billy Pilgrim exercise his own will at any point in the novel? If so, when?
Chew on This
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, perhaps it's the first and last 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 a nutterbutter. 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.