Five–The Importance of Being Earnest
As you said in the question, normally in a play that expresses social criticism we expect to find characters who oppose the social vices by standing for something

positive themselves. It seems in this play we find characters who express criticism of nearly every social vice and actually stand for nothing as they themselves perform, partake in, or exhibit most of the attitudes and behaviors they claim to oppose.
Many of the characters foreshadow the changes and actions they will undergo throughout the play with lines that suggest they believe or desire the exact opposite of it. First is Algernon, who establishes himself as an incurable bachelor by expressing his opinions on marriage very clearly to Jack, who intends to propose to Gwendolyn. Proposals are business, not pleasure, proposing, being accepted, and marrying are the opposite of romance to him, divorces are made in heaven and, “The only way to behave to a woman is to make love to her if she’s pretty, and to someone else if she’s plain.” If we were to know the formula of polar opposites from the start, we would know that this obviously means Algy will fall in love, become romantic, and marry happily.
The next character we see is Jack, who besides being a complete romantic, head over heels in love with Gwendolyn, is also a very accomplished liar, or Bunburyist. Jack we find out almost immediately, has presented himself to his acquaintances in town as Ernest, the nonexistent brother he pretends to have in order to leave the country whenever he wishes. Although he later claims to have no brother and never intends to have a brother, he eventually learns that he actually is this brother Ernest that he and later Algy, pretends to be. But of course not before he accuses Algy of being untruthful for copying this deceit. In a way, the only truthful things that Algy and Jack utter are those intended to be lies; as Algy says, “The truth is rarely pure and never simple.” Not in this play at least.
Next is Lady Bracknell, the hilarious embodiment of falseness, hypocrisy and inane social etiquette, such as; smoking is a good occupation for a man to have so as not to be idle, it is absurd for invalids to shilly-shally with the question of whether to live or die, ignorance is the preferred state of being, delicate and exotic, and not to be tampered with. Cecily’s hair on the other hand, is “Almost as nature might have left it. But we can soon alter that.” So from Lady Bracknell we learn the lesson that ignorance is as beautiful as a flower and should be left unspoiled, but hair–outer appearances, should apparently be quite the opposite. Because of course, “We live,” she regrets to tell us, “in an age of surfaces.” This makes Algy perfect husband material as he “has nothing, but he looks everything. What more can one desire?” Luckily for Algy, Cecily like his hair very much.
Gwendolyn, as the product of Lady Bracknell’s strict upbringing, says very little that means anything at all, but does it quite amusingly. Her first impressions are never wrong, as long as they may be changed at a moments notice, simplicity of character makes one exquisitely incomprehensible. Gwendolyn also provides insight to the proper and obviously very important beliefs of society; the right people have not seen spades, do not use sugar or eat cake, and of course, “In matters of grave importance, style, not sincerity is the vital thing.”
Lastly, we meet Miss Prism and Cecily. Cecily is the only character of the lot who is actually honest. Perhaps because she has been cloistered in the country, living a fantasy life through her diary, she hasn’t had the opportunity to learn what society deems important enough to lie about. Miss Prism on the other hand, is hiding a dark secret, although she seems to be the character least likely to have a dark secret to hide. Miss Prism follows the flip-flop pattern as well, with her foreshadowing comment, “I am not in favor of this modern mania of turning bad people into good people at a moment’s notice.” With the discovery that Miss Prism is guilty of child abandonment, and hiding out from the law, she herself is turned from a good person into a bad one, though not for long. All turns out well as lies are surprisingly revealed as truths, mysteries solved and marriage the happy ending for all.
How serious is this criticism meant to be? On the surface, in the age of surfaces, nothing is serious. On the surface, true love conquers all obstacles. On a deeper level Wilde is “trashing” this superficial society which is nothing but false and hypocritical, one in which appearances are everything. The same society which held him in such high esteem one moment, then shunned him the next when his “true nature,” i.e. his homosexuality, was exposed. Wilde mocks the heterosexual convention of marriage which was probably for himself and many of his male lovers, merely a facade men were expected, required, to participate in.