Yossarian initially seems to be as crazy as all the other characters in this madcap novel, but in reality, he is the most sane person of them all. Only a sane person would look at the mindless carnage he sees everyday and want to put an end to it. While his desire to go back home seems cowardly and self-serving at first, what Yossarian really wants is an end to this nightmare where he's putting his life and the lives of the others on the line for no reason at all.
Yossarian has the most grounded sense of morality as well. When he is given a chance to go back home a hero so long as he expresses public support for the extension of flight missions for the other men or face court-martial, he simply decides to escape to Sweden, which will label him a fugitive. However, he is not willing to compromise his principles by the taking the former option and endangering the lives of other people.
It is difficult to say then that Yossarian undergoes a character arc in terms of outlook or personality....
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