He could be regarded as sane in his own way as through his act of being deaf and dumb he observes everything going on around him and as a result notices the faults in those regarded as normal or sane. This is true of his views of society as well. All though he hallucinates and is obviously mentally unbalanced, the whole novel actually shows his progression towards sanity, paralleled with Randle McMurphy's regression into insanity. The escaping of Chief Bromden at the end of the novel shows that he has taken hold of his life and has finally made the decision to fight the opression that society has forced upon him.
Hope this helps a little bit.