In the prologue, Chaucer satirizes the prioress by having the narrator praise characteristics that are not representative of nuns. When we think of nuns, we think of women who have taken vows of chastity, poverty, and faithfulness to god. The narrator praises none of these qualities. Instead we hear that she sings through her nose; she speaks French (but not in the Paris style--in other words, not very well); she has nice table manners in that she never spills sauce on her bosom; she gives her animals the finest foods; and she is rather large. She wears a gold trinket that says "love conquers all." In other words, a Mother Theresa, she isn't.
This woman clearly enjoys the finer things of life. She enjoys a luxurious life. Instead of taking care of the poor, she is speaking French. Instead of feeding the hungry, she is eating very well--to the point that she is overweight. When so many people did not enough food to eat, feeding pets roasted flesh, milk, and fine white bread would have seemed very indulgent indeed. Her golden brooch would also have seemed out of character for a nun. It would be the equivalent of a nun today wearing a gold heart rather than a cross.
The method of satire is "damning by faint praise." In seeming to praise the nun's virtues, the narrator instead exposes her lack of commitment to her religious responsibilities and vows.
Her tale is no less revealing. It is an exaggerated miracle story showing her intolerance for differences and her narrow-minded thinking. The Jews are the evil villains in her story, and the miracle is so outlandish--a little boy continues to sing even after his throat is slashed--that it is preposterous rather than inspiring or credible.