Vonnegut doesn't actually make any direct statements about the war in the book, though of course he is obliquely trying to bring out the absurdity and some of the moral rot that surrounds war and the decisions people make and then justify during wartime. The centerpiece of the book is the fire-bombing of Dresden, something he witnessed first hand and something that had a profound effect on him.
There are many things he does that comment on the war, the willingness of the Tralfamadorians to not worry about people dying and the way that they can simply ignore the bad things and focus on the good in their ever-present world. But mainly I think he points to the absurdity of it through the way he tells the story.
Vonnegut's Slaughterhouse-Five is a satire aimed at criticizing war, particularly the bombing of Dresden during World War II. The bombing of Dresden is a controversial issue because many claim that the bombing was unnecessary. It was ordered by British and American forces because the area supposedly was a major transportation and communications hub; however, the bombing destroyed a large area and created a massive firestorm, killing many people and destroying a cultural area. Some argue that many of the buildings did not even hold communications equipment, making the bombing pointless. Vonnegut seeks to explore the pointlessness of the bombing through Billy Pilgrim's journey. Billy jumps back and forth in time--a statement to how the war continually returns to haunt Vonnegut--and experiences events repeatedly. Billy realizes that his life is absurd--a statement on the bombing itself.