Oscar Wilde's tone throughout The Importance of Being Earnest is satirical, ironic, and humorously critical.
He pokes fun at the Victorian preoccupation with moral improvement through characters like Miss Prism and Dr. Chasuble, and he lampoons the English aristocracy, with all its anxieties and prejudices, through the character of Lady Bracknell. Although The Importance of Being Earnest touches on some serious social and political issues, Wilde's criticism always remains light-hearted.
Wilde achieves this blithe tone with dialogue that is highly sophisticated and artificial. No real-life individuals are as clever as Algernon, who seems to speak entirely in witty epigrams and puns. As a result, the audience is always cognizant of the fact that they are watching a play and that the individuals on stage are not constrained by the rules of normal society.
The tone of The Importance of Being Earnest is also incredibly self-aware. Algernon, despite being an obvious self-insert for the author and for the Aesthetic Movement as a whole, is just as foolish and dishonest as the other characters. By making fun of the English upper class, Wilde must also make fun of himself, and he does not shy away from doing so.
This line from Lady Bracknell perfectly encapsulates Wilde's comedic, self-aware tone:
Lady Bracknell: Algernon is an extremely, I may almost say an ostentatiously, eligible young man. He has nothing, but he looks everything. What more can one desire?