There are many words that could be used to describe Orgon: idiot, dunce, sucker, chump, chucklehead, chowderhead, dunderhead – hmmm, what's with all these head words…? Anyway, you get the drift. Orgon doesn't exactly behave well in Tartuffe. Any guy who claims, "My mother, children, brother, and wife could die,/ And I'd not feel a single moment's pain" has a serious screw loose as far as we're concerned (1.5.4). Furthermore, any guy who cares more about the "holy" man who taught him to feel that way than he does about his family has some really serious issues.
Now, it's easy enough to write Orgon off as an idiot, a dunce, a sucker, etc., but there is a little evidence that he has (or had) some brain cells. As Dorine tells Cléante in Act 1, Scene 2,
In the late troubles, he [Orgon] played an able part,
And served his king with wise and loyal heart,
But he's quite lost his senses since he fell
Beneath Tartuffe's infatuating spell. (1.2.4)
The key words here are "wise" and "but." "Wise" because it suggests that, at some point Orgon was not simply competent, but actually insightful. "But" because it points right at the cause of his recent transformation: "Tartuffe's infatuating spell." The rest of the family probably wouldn't be so committed to saving Orgon if he'd always acted like a fool.
You could even see Orgon's foolishness as evidence of his basic goodness. For example, he agrees to help out his friend, Argas, by storing the incriminating papers. That's not the smartest thing to do, but it does show loyalty. He's also incredibly loyal to Tartuffe, sticking with him until it's painfully obvious that the holy man is a fraud. This, of course, is really foolish, but it does show that Orgon stands behind his friends.
For all his faults – his gullibility, his tendency to fly off the handle (Ever wonder where Damis got that from?), his pigheadedness – he's a good guy. It's just that sometimes, he needs a little help from his friends. He only falls for Tartuffe's tricks because he's too willing to believe in Tartuffe and all his claims. After all, Tartuffe promises him the path to piety and salvation. Can you blame him for buying into it? Well, maybe a little.