Because Slaughterhouse-Five is an anti-war book, Vonnegut isn't presenting us with any heroes. And to counteract the impression that any of the men in the novel have the self-determination or free will to make heroic choices, Slaughterhouse-Five relies a lot on absurdity. Billy Pilgrim is described repeatedly as clownish; he looks so ridiculous that a German surgeon on the streets of Dresden criticizes him for making a mockery of war. Paul Lazzaro and Roland Weary are both so self-absorbed that they don't even seem to notice that they are on a battlefield half the time. And even poor Edgar Derby, who is so idealistic and committed, can be reduced to tears by the unexpected taste of syrup in his mouth by the end of the war.
Still, the real foolishness in the book is not at the individual level. We can't help but think there must be something wrong with a system that would send poor, innocent Billy Pilgrim to war.
Edgar Derby's belief in justice may be foolish, but the narrator also expresses sympathy for his efforts to resist a system he believes is wrong
Billy Pilgrim continues to appear clownish throughout his entire wartime experience because the narrator explicitly wants to describe World War II as a Children's Crusade. Billy is the ultimate foolish innocent sent into a battle he does not understand.