Orgon may be the king of fools in Tartuffe, but he's not the only one that's off his rocker. As Dorine points out, Mariane and Valère are suffering from a bit of what the French call l'amour fou (mad love). And, indeed, Tartuffe's undoing is his own foolish love for Elmire. Unfortunately for just about everyone involved, this kind of folly is catching; more often than not, the fool is under the impression that he is the only sane person left. In Tartuffe, that rule holds true as ever.
Orgon's foolishness, though troublesome, springs from a deep, if naïve, trust in his fellow man.
Orgon represents little more than a bundle of incorrect opinions waiting to be refuted by the other characters.