Tone is a highly important element in this novel. It is also, however, a rather complex element of the novel.
Vonnegut employs a tone that can be described as wry, bitter, and even outraged. Yet it is also compassionate, empathetic, humorous and resigned.
Vonnegut uses black humor throughout Slaughterhouse-Five (...) to reinforce the idea that the horrors of war are not only tragic, but inexplicable and absurd. (eNotes)
The repeated phrase, "So it goes," demonstrates a sense of "learned detachment" that pervades the narrative. For a novel that is seeking to expose and instruct on certain facts from a war while also roundly condemning conventional views of warfare (among other things), the author must take care not to fall into a polemical tone. If the book appears to be attempting primarily to prove a point or make an argument, it ceases to be fiction and becomes merely analogy or, worse, lecture.
For this reason, tone is very important and Vonnegut's successful use of a complex tonal voice provides a sense that the writer is invested in a point of view but not pressing a point/argument onto the reader. There is enough distance from the outrage and enough humor to buoy the atmosphere of the novel, balancing bitterness with acceptance; judgement with compassion.