It has been ages since I read the book or watched the movie, however my understanding has always been that McMurphy conned his way into a mental institution to avoid what he imagined was the harsher reality of serving a term in prison.

I think it is the juxtaposition of a 'sane' person held in a psychiatric ward that gives the audience a lens through which to view the conditions of the institution and possibly how McMurphy was caught up in his struggle against the system.