The Free Dictionary defines a comedy of manners as a "witty, ironic form of drama" that ridicules, or pokes fun of the "manners and fashions of a particular social class or set." The plot must also deal with "some scandalous matter" (comedy of manners, The Free Dictionary by Farlex). In the case of Pride and Prejudice, using a scandalous marriage between Elizabeth and Darcy, Jane Austen pokes fun of the demarcations of class distinctions, the arrogance of the upper middle-class, and the vulgarity of the rising middle-class.
Austen uses the relationship between Elizabeth and Darcy to poke fun of the commonly restricted social interactions between nobility and the upper middle-class. Though Darcy is only a gentleman and not titled, he has noble relations, defining him as nobleman. In contrast to Elizabeth, whose father is also a gentleman, Elizabeth has a few working middle-class relations, such as the Gardiners and the Philipses. Hence, Elizabeth is not considered as high on the social scale as Darcy, making their marriage scandalous. Austen wrote the novel not long after the French Revolution which had a dramatic impact on the British aristocracy. Not wanting to find themselves likewise beheaded, the British aristocracy began acting less arrogant and mingling and intermarrying with the English middle classes. Austen's novel serves as a loose commentary on the heavy lines society was previously drawing between classes. Elizabeth's scandalous marriage to Darcy served to help erase some of those heavy social lines.
Austen uses Darcy's initial condescending attitude to poke fun of the commonly accepted arrogant attitude of the aristocracy. She also uses Lady Catherine de Bourgh to further poke fun of this social concern.
Finally, Austen uses Elizabeth's middle-class relations to poke fun of the vulgar manners of the working-middle class. Mrs. Bennet frequently displays vulgar manners with respect to saying ridiculous things in public and gossiping. Her working-class sister, Mrs. Philips, is guilty of the same vulgarity.