Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Sanctimonious, Emphatic, Humorous
The tone of the Pardoner's Tale definitely fits dictionary.com's definition of "sanctimonious": "making a hypocritical show of religious devotion, piety, righteousness, etc." The Pardoner rages against lots of different sins, despite the fact that he's guilty of all of them. He preaches a long sermon against drunkenness despite having insisted that the pilgrims stop at an alehouse before he told his tale, and is likely drunk while telling it. He calls gluttony "ful of cursednesse," and blames it for mankind's damnation, yet has just admitted to the pilgrims that he's willing to take a widow's last penny to buy cheese, wheat, and wine for himself.
Compounding the hypocrisy is the sheer intensity of his attack. A favorite tool of the Pardoner is "apostrophe," or address to an abstract concept or inanimate object, emotionally delivered. "O glotonye," he says to Gluttony, "ful of cursednesse!" He gives the same treatment to drunkenness and oath swearing. The heightened emotion makes it all seem that much more sanctimonious.
And he's pretty funny. We can't deny that the Pardoner's sermons can be hilarious because of what we know about his own character. But he's a great comic storyteller, too. His descriptions of gluttony, his explanations of the tricks of his trade, his over-the-top language—they're all comic despite the "serious" content.