Cléante is both a wise man and a wise guy: he's a perceptive, learned, intellectual, you name it. He's also a big talker, one who never seems to doubt the truth of anything he says. Cléante spends most of his time telling the admittedly dense Orgon how wrong he is about everything. Cléante is right about so many things that it would be impossible to list them all, but there is a section of dialogue that illustrates the two sides of Cléante well. Be prepared: it's a long section. But how could it not be? It's Cléante talking after all.
Ah, brother, man's a strangely fashioned creature
Who seldom is content to follow Nature,
But recklessly pursues his inclination
Beyond the narrow bounds of moderation,
And often, by transgressing Reason's laws,
Perverts a lofty aim or noble cause.
A passing observation, but it applies.
I see, dear Brother, that you're profoundly wise;
You harbor all the insight of the age.
You are our one clear mind, our only sage,
The era's oracle, its Cato too,
And all mankind are fools compared to you.
Brother, I don't pretend to be a sage,
Nor have I all the wisdom of the age.
There's just one insight I would dare to claim:
I know that true and false are not the same. (1.5.9-11)
Now, we're going to cut Cléante off here, but just know: he goes on and on explaining his "one insight." All of what Cléante says here is pretty much correct: mankind is definitely capable of perverting a lofty aim or noble cause. Tartuffe is a prime example of that.
However true the things he says may be, Cléante has a knack for presenting them in an annoying way. The guy is really just downright pretentious at times. Look at how he ends his remarks by saying, "A passing observation, but it applies" (1.5.9); or how he insists that he's not a sage or at all wise, but…. It's enough to make you want to side with Orgon in this case. Sure Orgon's comments are pretty much meaningless, sarcastic fluff, but even the wisest, noblest members of the audience may want Cléante to shut up at some point.
The fact is, all Cléante's wisdom doesn't have a great effect on Orgon. Even when Cléante foregoes the intellectual stuff and attacks Orgon straight on he's ineffective. He yells at his brother-in-law, "Good God, man! Have you lost your common sense – / Or is this all some joke at my expense?" (1.5.7). Still, the bull-headed Orgon refuses to listen.
Of course, at the end of the play, Cléante finally gets through to his brother-in-law. After, Tartuffe has been exposed and packed off to jail, Orgon goes off on the false holy man. Cléante steps in and once again tries to make his rash brother-in-law behave himself. "Ah, Brother, please," says Cléante , "Let's not descend top such indignities./ Leave the poor wretch to his unhappy fate…[and] hope that he/ will soon embrace an honest piety" (5.7.25). In the end, Orgon finally listens to his wise brother-in-law, giving us some hope that the family will live with a little more moderation in the future.