The relentless wisecracking IMO is best thought of as a combination of self-defence in an insane situation (that gets progressively more insane) and as a sort of dissociation. There is an element of humanity in it "I laugh that I do not weep" and also a philosophical sense that a sense of humour is the final sense of proportion that can remain to us, to maintain our sense of order in a world filled with chaos.

There is a very unfunny central narrative involving Yossarian's attempts to get out of the Air Force and a crucial PTSD incident that his brain keeps revolving around, that is only gradually revealed, and hence hangs around forever, as the cause of his irrational behaviour.

Paradoxically, it's morally a very serious novel. It's very anti-war, even anti-patriotic. There is a scene in which Yossarian is raging against the witless moralistic patriotism of one of his co-officers who is talking about "the enemy" and Yossarian tells him that the US generals are the enemy, that "the enemy is anyone who is trying to get you killed". It was quite a bleak view of human nature, but it was also humane.

I think it's useful to remember that although it is set in WW2, the ethos it is describing is one informed by events from 1953 when he started writing it, to 1961 when he got it published.

It isn't really about fighting the Nazis, it's about a cult of inhumane behaviour justified by the idea of protecting the nation from Evil; it's about McCarthyism, it's about pointless battles for personal prestige and enrichment being fought by mediocre little men. It's about disillusionment.