There is a definite sense in which this could be argued. Yossarian could be viewed as the only believable character in this novel because he is the only one who seemingly manages to understand clearly what is going on and does everything he can to avoid being caught up in a war that only dehumanises the soldiers and is a catastrophic waste of life. Yossarian, although he is thought to be mad by his fellow soldiers, actually seems to be one of the very few characters who sees life for what it really is, and one of the clear messages of the play is about man's mortality and the eventual death that we all face, even Yossarian. No wonder then that he does everything he can to escape the war and live life as richly and as fully as he can. Note how the concept of mortality is conveyed through Yossarian's repeated reference to the death of Snowden. This following quote is taken from Chapter 41:
Yossarian was cold, too, and shivering uncontrollably. He felt goose pimples clacking all over him as he gazed down despondently at the grim secret Snowden had spilled all over the messy floor. It was easy to read the message in his entrails. Man was matter, that was Snowden’s secret. Drop him out a window and he’ll fall. Set fire to him and he’ll burn. Bury him and he’ll rot, like other kinds of garbage. That was Snowden’s secret. Ripeness was all.
Yossarian is therefore the most believable character because he manages to accept a truth that none of the other characters are able to see or comprehend: that man is inherently fragile and doomed to die. Or, as Yossarian realises, "Man was matter." As a result, the only possible way to live one's life on the basis of this truth is to realise that "Ripeness was all." In spite of the rather grim and pessimistic message, Yossarian realises that it is possible to enjoy fullness of life, however briefly, and it is this awareness that leads him to just walk away from the war, as he sees the war as the enemy of "ripeness." It is this clarity of understanding of certain essential truths that makes Yossarian the most believable character in this novel.