Comedy tends to rely on contrast. What makes something funny is the disconnect between the environment and the actions of the people in that environment. For instance, in the 2011 comedy Bridesmaids, some might find the food poisoning scene at the bridal shop funny because the behavior of the bridesmaids contrast with the elegance of the store.
Something similar could be said about the effects of the rhyming couplets in Molière’s two plays The Misanthrope and Tartuffe. When the characters speak in rhyming couplets, they draw attention to the dichotomous situations that make the plays funny. When words rhyme, they, in a sense, go together. One could make the argument that Molière uses rhyme to reveal the opposite. The rhymes make it clear to the audience that his character’s aren’t in sync but out of sorts.
Think about the opening scene in The Misanthrope between Alceste and his friend Philinte. At one point, Philinte tells Alceste (in Richard Wilbur’s translation), “With all respect for your exalted notions, / It’s often best to veil one's true emotions.” This couplet packs two competing ideas into one rhyme. It pairs the notion of honesty with the practice of hiding one’s feelings. The discordant pairing should aid in its comedic effect.
Such comedic tension ties into Tartuffe. The rhyming couplets belie the clashing views on Tartuffe. Early on, Dorine tells Orgon’s mom (also in Wilbur’s translation), “You see him as a saint. I’m far less awed; / In fact, I see right through him. He’s a fraud.” Once again, the tight rhyme scheme shows the disjunction at the center of Tartuffe and thus reinforces the incongruous amusement.