I suppose you could label this work science fiction, even though the elements that make it so are passé and unsophisticated. The sci-fi angle is one Vonnegut uses to excellent effect. It points out how otherworldly a war scene can feel, how memories can be real enough to throw the time gamut out of synch, and how some superior intellect in the universe can explain, almost deterministically, why things are as they are and people do as they do.
One question that must be pondered following a reading of the work is: does Vonnegut himself truly believe in the free will of the individual even though his characters suggest otherwise. I believe he does and merely uses the characters in the novel as counterexamples to his true feelings.