Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Satirical, Ironic, Light
Ironic
Now, we tread lightly wherever we use this word, because we don't want to end up like Alanis Morissette, but this play is ironic. And we're talking in the real sense of the word, where what people say is the opposite of what they mean.
Take Célimène. Almost everything that comes out of her mouth is ironic, especially when she is talking to Alceste. Like, when she says to him, "Your love for me is matchless, Sir; that's clear" (2.1.77), she doesn't mean that's a good thing. What she really means it that no one has ever loved her like this, and thank goodness for that!
Or, when Arsinoé comes to tell Célimène how she feels under the guise of friendship, Célimène repeats Arsinoé's words back to her with an ironic twist: "Madam, you're too intelligent,/ I'm sure, To think my motives anything but pure/ In offering you this counsel—which I do/ Out of a zealous interest in you" (3.3.82).
Célimène is obviously saying these things not because she is Arsinoé's friend, but because she doesn't like her. Unlike Alceste, Arsinoé is savvy enough to get the insult—and she's pretty offended.
The thing is, this irony isn't just funny—it's necessary. If you just tell people what you think, like Alceste does, you're going to end up with a lawsuit. But if you use irony, and your words don't mean what they seem at face value, you can always cover your tracks. Célimène could always say that she totally meant that she was bestest friends with Arsinoé—and it's really hard to take someone to court over someone's tone.
Satirical
If Molière were living today, he probably wouldn't be writing plays; he'd have a late-night show on Comedy Central. Satire is when you pick out the worst parts of something, someone, or society itself and make fun of it on stage.
It's kind of like saying, look how silly you look, don't you think you can do better? While Stephen Colbert satirizes right-wing (and left-wing) pundits and the whole format of news shows in general, Molière is taking a swing at French nobility. (For more on this, tune into the "Genre" section.)
Light
Even though he's got serious reforms on his mind, Molière doesn't take himself too seriously. Come on, how can a guy who wrote a scene that only includes these lines be that serious of a dude?
CELIMENE: What is it?
BASQUE: Acaste is here.
CELIMENE Well, send him in. (2.2.1)
And that's not the only short, hilarious scene. Sure, there are some heavy moments, but Molière never gets bleak or depressed. He may think people are pretty silly, but, unlike Alceste, he hasn't given up on them in despair.