Is it unprofessional to include a humorous quote in a PG thesis?
Unless your advisor and committee are strictly against it, there should be no problem with including a quote like this somewhere.  Formats for theses vary, and sometimes a preface or the introductory material can be presented in a fairly conversational way.  That would be a perfectly appropriate place for such a quote, as would the acknowledgements.  However, I have also seen humorous or literary quotes inserted at topical points into the body of a thesis, sometimes with several of them sprinkled through the body text.  In what can often be a very dry read, they may help to liven up the document.

However, as I noted in my first sentence, while this is generally fine, you don't want to do it if it will run afoul of the people responsible for grading and accepting your thesis.  Sometimes readers can be unnecessarily stodgy about these sorts of things, so it would always be a good idea to check how your readers feel first.