"The Miller's Tale" is largely a reaction to the tale before it, "The Knight's Tale." "The Knight's Tale" is a chivalric romance depicting a love triangle between a fair princess and two knights. There, all three of those characters were virtuous in one way or another. Not so in "The Miller's Tale," where all of the characters are either foolish or wicked. Everyone is in some way unpleasant and grotesque.
The description of the miller's wife Alisoun is a great example. The lengthy description of her emphasizes her beauty but in ways that also illustrate her nastier characteristics: her body is described as being as small and graceful as a "weezul"—weasels are not normally associated with virtue, but with deceit, paralleling Alisoun's own penchant for adultery. She is also compared to a colt, which emphasizes her youthful, untamed energy.
Nicolas and Absalon both go after Alisoun, but only Nicolas wins her. Nicolas is direct, asking for sex upfront, while Absalon tries to woo her according to the courtly love tradition by singing her ballads outside her window. Yet despite their differing methods, both men are only inspired by lust. Neither loves Alisoun for her personality, but only for her body. Absalon only seems more foolish because he is pretending his lust is something better than what it is.
Unlike "The Knight's Tale," the ending does not emphasize honor or love. Instead, all of the men in love with Alisoun are punished: Absalon has his face farted at, Nicolas is branded on the behind with a poker, and John the carpenter is humiliated and injured. Unlike in "The Knight's Tale," which inevitably ends with one of the noble knights winning the princess, no one "wins" Alisoun at the conclusion of "The Miller's Tale." In fact, no one wins anything.