The medieval audience this story was written for would have been better able to appreciate the humor in the context of that era, and perhaps a more "rustic" life allowed for certain behaviors to be overlooked.
While the author praises Alisoun—particularly how she looks —I find Alisoun's character questionable, regardless of her "country manners," in Chaucer's "The Miller's Tale."
While it may not have been uncommon in a "fabliau" (which is the form Chaucer uses in telling this tale) for the satirical nature of the literary piece to poke fun—contrary to the author's admiration of Alisoun's physical "attributes," I find it hard to admire Alisoun based on her behavior.
First of all, she is married, and while John may not be as young, smart or handsome as she might like, and he is annoyingly jealous, Alisoun has made a commitment before God to him, which she ignores when she agrees to have an affair with Nicholas.
We learn Alisoun is young and newly wed; her husband loves her more than his life, and is fearful that she might have an affair behind his back ("cuckold"):
This carpenter had lately wed a wife
Whom he loved better than he loved his life;
And she was come to eighteen years of age.
Jealous he was and held her close in cage.
For she was wild and young and he was old,
And deemed himself as like to be cuckold. (35-40)
One would expect that with such a new marriage, Alisoun would be faithful to her new husband for a little while, but she obviously does not intend to be devoted to her husband. Chaucer notes:
And certainly she had a lickerish eye.
This means she has a "lecherous" eye, and "lechery" is defined as...
...unrestrained or excessive indulgence of sexual desire.
The reader (and John, it would seem) can have little hope of her fidelity to her new and loving husband. And when John travels from their home, Nicholas is there waiting.
Alisoun is easily attracted to Nicholas, though looking at the description objectively, he seems lacking:
This clerk was called the clever Nicholas;
Of secret loves he knew and their solace;
And he kept counsel, too, for he was sly
And meek as any maiden passing by. (13-16)
Nicholas gets around ("secret loves") and he's sneaky and "sly." Had Alisoun been a good wife, she would have done everything to stay away from Nicholas, however, she quickly falls into a relationship with him.
Besides turning away from her husband, she is particularly critical of a parish clerk of the church named Absalom who also wants Alisoun. She will have nothing to do with him because she has promised herself to Nicholas (not out of desire to be faithful).
She was enamoured so of Nicholas
That Absalom might go and blow his horn;
He got naught for his labour but her scorn.
And thus she made of Absalom her ape,
And all his earnestness she made a jape. (199-203)
Here is a young woman without regard for others. She also has no class.
Ultimately, Alisoun and Nicholas find a way to trick John by making him think floods are coming. Alison plays a trick on Absalom by putting her naked bottom out the window for him to give a closed-eyed kiss, thinking it her face. When John falls through the floor of his house in a boat constructed for the "floods," he is a laughingstock. Absalom leaves in disgust, and Nicholas is branded on his butt by Absalom.
However, nothing happens to Alisoun. It seems unfair, but this is just one characteristic of a "fabliau," where the husband is played for a fool by his wife.