This is of course one of the central questions concerning this novel. It is important to remmber that Yossarian is the central character that is used by Heller to express his own thoughts and feelings about war. Yossarian is a character who is obsessed with trying to avoid missions, and finds it increasingly frustrating that the number of missions he is expected to complete rises rather than falls. On the one hand, his feelings about war and his determination to question and challenge his superiors could lead us to brand him a coward. However, this would be but a very superficial reading of this hilarious and brilliant story, because Yossarian is definitely not a coward, as is shown when he goes AWOL and through his constant challenging of his superiors. When we consider how Yossarian deliberately disobeys orders and ruins missions, we cannot think of him as a coward.
Instead, Yossarian is a humanitarian character who is well aware of the futility of war and how so many of the actions that soldiers engage in are actually pointless. This of course leads to one of the central conflicts in the novel, as Colonel Cathcart becomes incredibly angry that Yossarian does not display the kind of values that the military wants to see in its soldiers: patriotism and blind obedience. Yossarian shows throughout the novel that he is a set of values and morals that he cannot conveniently put aside for the duration of the war. This is why he thwarts various missions, making the squadron bomb the sea rather than an Italian town which represents no military danger. He is a character who is shown to hate war, and is particularly suspeptible to its horrors, as the way that he constantly replays the death of Snowden shows. However, what distinguishes him from other characters is that he acts on his feelings. We cannot therefore call him a coward.