Through the use of his naive narrator, Chaucer actually praises all of his characters. Even the Friar and the Pardoner are praised. The narrator doesn't know any better. He thinks all the characters are fantastic.
Thus, Chaucer, writing with irony, leaves the conclusions up to the reader. Though the narrator thinks it positive that the Friar finds mates for the women he impregnates, the reader draws an opposite conclusion. Though the narrator reveals the cleverness of the Pardoner, who uses a tale about greed to feed his own greed, the reader sees the irony.
Chaucer's narrator presents positive views of the characters, enabling Chaucer to use irony to reveal the truth to the reader.
Specifically, one of the main criticisms Chaucer levels against his society is church corruption. You can look at the Friar or the Pardoner for positive characterizations that use irony to reveal corruption. These corrupt characters are presented just as positively as the characters who are not corrupt, but they are presented ironically. That's how Chaucer presents his criticisms.
Chaucer is considered a student of human nature. He was very observant about what he saw transpiring around him and used his writing to convey praise or disapproval.
While Chaucer was very unhappy with the hypocrisy he noticed taking place around him (especially with members of the Church), he was quick to praise those who were humble, caring, honest and giving.
In particular, one character he praises is the Knight. He is a humble man who has served in battle with valor. He has been to the crusades, he does not dress to impress, but the clothing beneath his armor is worn from extensive use. He is humble and well-mannered, and in coming home, his first obligation is to to go on a pilgrimage to give thanks.
Praise that Chaucer would offer by inference regarding the Knight would include advice that a knight would have to honor his rank by fighting, and fighting well. This kind of a man would be expected to handle himself as a gentleman, not try to impress people with fancy clothes, and humbly present himself at a holy place to give thanks to God for his safe return.
By praising the Parson, Chaucer draws particular attention to his strengths, and by addressing the righteous things the Parson does, he draws attention to behaviors that are directly hypocritical to God's "calling," specifically with other men of God such as the Monk and the Friar.
The Parson is a man who has taken his vow of poverty seriously (as the Monk and Friar have not). Everything he teaches comes straight from the scriptures for the benefit of his parish. He is patient in the face of difficulty. He does not ask the poor members of the parish to give him money (as do the Monk and the Friar); instead, the Parson would give to the poor from his own "goods" and "Easter offerings."
The Parson would work tirelessly for his "flock," never failing to visit those sick or in need, despite the poor weather. He was not interested in entertaining himself or traveling for fun, but stayed close to home to be of service to his "sheep."
The Parson is very different than the Monk or the Friar who hunt, travel, take money from the poor, and are intimate with the women. (The Monk does some of these, the Friar the others.) The Parson welcomed all men and women regardless of their lot in life, and lived a life that set a good Christian example.
By extolling the virtues of the Parson, by example he points out what is admirable in a man, and what is worthy of censorship (as with the Monk and the Friar).