Symbolism, Imagery, Allegory
It shouldn't come as much of a surprise that the imagery in Slaughterhouse-Five is topsy-turvy. Actually—let's rephrase that. This entire book is an exercise in weird, genius repetition and mixed-up reality: this entire book is a surprise. That the imagery is similarly wackadoodle is slightly less of a surprise.
A lot of the imagery in Slaughterhouse-Five repeats across sections and in different contexts. For example, the narrator describes his own breath when he is drunk as "mustard gas and roses" (1.3.2)— which is what his dog, Sandy, specifically does not smell like (1.4.14).
This is also the odor of the corpses at Dresden a couple days after the firebombing, which Billy Pilgrim discovers as he digs through the rubble of the city in Chapter 10. This repetition of description serves to connect the "Billy Pilgrim" portion of the novel with the narrator's own personal memories and experiences.
Other examples of repetition of imagery include descriptions of characters "nestled like spoons": Billy and the hobo/private in the prison boxcar (3.29.3), Billy and his wife Valencia (4.1.2), and the American soldiers on the floor of the shed in the British compound (6.10.1) all nestle like spoons as they sleep. Humanity may be super-weird and super-diverse, but everyone likes to spoon.
There is also Billy's "ivory and blue" (4.1.4) bare feet as he walks barefoot through his Ilium, New York home, the "blue and ivory claw" (4.5.1) of his cold hand clinging to the vent in his boxcar on the way to a German POW center, and the "blue and ivory" (6.16.4) feet of the dead hobo lying outside the train that will take Billy to Dresden. Again: humanity may be super-weird and super-diverse, but everyone's feet are gross.
The repetition of these phrases—mustard gas and roses, nestled like spoons, and blue and ivory—demonstrates that no part of this story is isolated from any other. Each section, as brief as it may be, fits into a larger consideration of war and its aftermath. So it goes, eh?