One of the themes of Vonnegut's novel relates to the idea of perspective and interpretation. Text is a natural medium for dealing with this concept.
Some aspects of the novel that express this theme are Derby's death and the under appreciation of the significance of the bombing of Dresden.
Derby is killed for looting after Dresden is bombed. He is killed by firing squad at the end of the novel, despite the fact that over 100,000 people had just been killed by fire-bomb and there was wide-spread looting throughout the war and in Dresden as well. Derby's theft was interpreted/perceived as a crime while the bombing was considered a justified act of war. Derby is killed. The people who ordered the bombing are not even in the field of war to be judged or to answer for their decision.
This is not irony, not exactly. Rather, this situation indicates the novel's thematic message, which suggests that what we see is determined by how we are looking. Reality is the product of perspective.
While the Hiroshima bombing is considered to be the biggest bombing in WWII, the fire-bombing of Dresden actually leads to more fatalities. This is another example of how reality is shaped by perspective.
The bombing raid created a firestorm that destroyed the city and killed an estimated 135,000 people, almost all of them civilians. This was nearly twice the number of people killed by the first atomic bomb dropped on Hiroshima. The Dresden bombing remains the single heaviest air strike in military history. (eNotes)
The narrative of WWII often "favors" the view that Hiroshima and Nagasaki were the largest bombings in history. This is not factually true according to Vonnegut's novel, yet it remains "narratively true".
Perspective is at the heart of the novel and can be seen here again in this passage describing Billy's experience watching a war film in reverse. Seen in reverse, the war is restorative and peaceful:
"The bombers opened their bomb bay doors, exerted a miraculous magnetism which shrunk the fires, gathered them into cylindrical steel containers, and lifted the containers into the bellies of the planes.… The steel cylinders were taken from the racks and shipped back to the United States of America, where factories were operating night and day, dismantling the cylinders, separating the dangerous contents into minerals … [which] were then shipped to specialists in remote areas. It was their business to put them into the ground, to hide them cleverly, so they would never hurt anybody ever again."