Hypocrisy is a slippery thing. To some, it's obvious: Tartuffe is called a hypocrite pretty much right off the bat. The more we're told about him, the more noticeable his hypocrisy becomes. At the same time, hypocrisy goes hand in hand with deception; it represents an effort to project a false image. It's a hard act to pull off, and Tartuffe doesn't do a particularly good job of it, but he still succeeds in putting everything off balance. That's the real problem with hypocrisy: it calls the truth of everything into question.
Though Tartuffe definitely doesn't practice what he preaches, he manages to deceive himself; he actually buys into some of his lies.
By putting a hypocrite at the center of the play, Molière puts everything off balance. We're forced to question every statement and every action, to decide if they each – or any – should be taken at face value.