What does Chaucer think about the nun?

The place to construct the answer to this question is in the general prologue, the part of The Canterbury Tales where Chaucer introduces the characters.
From the outset, it is clear that Chaucer finds the nun to be a fake, even using the term "counterfeit." Her smile is "ingenuous and coy," and her manners are noted to be well-practiced, while her French was perfect but not the French spoken in Paris. The nun gives the impression of someone trying too hard to put on airs. When Chaucer mentions that her "only oath was to Saint Loy," the reader is meant to understand that the nun, like Saint Loy (a Merovingian-era saint who wouldn't take an oath) cannot be trusted to take an oath at all.
Further, Chaucer gives us clues through the name and titles he assigns to her. Being named Eglantine, a word derived from the word "elegant," hints at her worldliness, while referring to her as "Madam," as opposed to "Sister," is meant to lead the reader further in that interpretive direction.
She dresses elegantly, eats well, keeps personal pets, and wears a fancy rosary with the inscription "love conquers all." All of this underlines Chaucer's judgment that this woman has not entered religious life for the sake of piety but instead is using religious life as a means of achieving various pleasures and social standing.