The Tralfamadorians are pretty clear that their novels hold no moral lessons for readers. After all, what would be the point of a moral lesson when you can't do anything to change the future? Slaughterhouse-Five, with its stars and tiny sections, seems to be imitating a Tralfamadorian novel. So it makes sense that the narrator doesn't spend much time preaching about right or wrong: that's not the point of this book.
If the book were really trying to deliver a moral message, the narrator's emphasis on the suffering of the Germans in Dresden might have to be balanced out by a much longer meditation on the Nazis' concentration camps. What Vonnegut seems to be asking his readers to do instead is to think about how much human suffering the war brought for both sides. Some of the most evil characters in the book – Bertram Copeland Rumfoord and Paul Lazzaro – are the ones who think they are absolutely right. This kind of righteous self-assurance is what leads to war in the first place.
The narrator tells his sons in Chapter 1 that they are not to participate in massacres, but Billy Pilgrim willingly sends his son Robert off to Vietnam, presumably to kill people. This difference between the narrator's and Billy's choices exposes a fundamental difference between the two men's characters: Billy is resigned to war, while the narrator is trying to prevent fighting.
By focusing on the suffering of individual human beings, such as the German refugee girls killed in the Dresden firebombing, Vonnegut shifts attention about the morality of war away from big questions of national politics toward smaller, less justifiable instances of personal pain.