I'll answer your question with a question: what is the purpose of an essay's title?
Maxine Hairston and Michael Keene, scholars of rhetoric and communication, identify four main purposes of an essay title:
First, it predicts content. Second, it catches the reader's interest. Third, it reflects the tone or slant of the piece of writing. Fourth, it contains keywords that will make it easy to access by a computer search.
Looking at those four purposes there is nothing there that cannot be accomplished by a question.
For a stellar example, consider the following essay title: "Is the Internet making us Dumber or smarter? Yes." (Note that it does use punctuation, to let us know that it is asking a question.)
First, it tells us what we will be discussing the internet and its effect on intelligence and education.
Second, it is an amusing conundrum presenting seemingly self-contradictory information, inciting curiosity in the reader to get the authors full opinion.
Third, the answer "yes" to an "or" question is goofy and irreverent, letting readers know that the topic will be handled with levity.
Fourth, it shows up very high on Google's search algorithms if you look for simple questions like, "is the internet making us stupid?".
So as you can see, questions can serve as effective essay titles. Unless your instructor's particular rules or style guide explicitly disallows it, you should have no trouble with a question mark on your title page.