The key theme of superficiality in Lady Windermere's Fan is illustrated by a cluster of subtopics that include keeping up appearances, keeping up with the social expectations of class, and the hypocrisy of moral double standards.
Granted, these are typical themes in the genre of comedy of manners, which portrays, in a satirical way, the overzealous preoccupation of Victorians to appear successful, well-mannered, and "proper" in the eyes of society.
It can all be summarized in Lord Darlington's words:
If you pretend to be good, the world takes you very seriously. If you pretend to be bad, it doesn’t. Such is the astounding stupidity of optimism.
Wilde, the master of this genre, presents these themes from the very start of the play.
Superficiality and keeping up appearances are essential themes in Lady Windermere's Fan because they cause nearly every action in the play.
For example, right from the very start, we find Lady Windermere engaged in putting her roses together. The way it is described, one would feel almost jealous of this lucky lady. She is a young, beautiful, and rich woman, who describes herself as someone who knows right from wrong and prefers people to be "proper" without any excuse. And yet, later on in the play, we will see her deviating from her moral compass and contemplating an elopement with another man.
The themes are also evident with Lord Windermere, the Lady's husband. He has one key action in the play: to protect his wife's social persona. He does this by keeping her from learning that her real mother, Mrs. Erlynne, is not only alive but also a mean, socially shunned woman who is planning to expose who she (Erlynne) really is. This would cause Lady Windermere a lot of shame. The appearance of domestic perfection that she embodies would come crashing down.
Lord Windermere also has to watch out for his own reputation. After all, according to the gossipy Duchess of Berwick, Society (with a capital S) "knows" about his dealings with the Erlynne woman. Moreover, Society "has been talking about it" behind the Windermeres' back, apparently having a field day with the situation.
On the other side of the spectrum, there are the characters who are shunned, or should be shunned, from society and yet try very hard to go back to it or continue to keep appearances.
The first is Mrs. Erlynne, Lady Windermere's real mother (unbeknownst to Lady Windermere). The woman was shunned from the "good" society for abandoning her husband and daughter to elope with a man. Still, she wants to be back in that circle. She resorts to blackmail and threats to Lord Windermere as ways to buy her silence. However, she still wants to disclose the truth to her daughter, even though she knows how insulting it would be for the sanctimonious Lady Windermere to find out who her mother is.
The second is Lord Darlington, who is known by everyone to be a dandy and "vile" as far as his morals go. He is still accepted in society only because his name, rank, and relations make him, by Victorian double standards, a member of the "polite" society. Hence, he keeps up the appearance of someone who is of a higher social bracket—but only in name and position. If he had no rank or position, he would probably receive the treatment that he really deserves, and nobody would go near him.
Therefore, the society in which Lady Windermere's Fan is set is very duplicitous. This is a time in which people presented themselves with fake and superficial appearances of righteousness and self-importance. As such, keeping up with appearances and with the social expectations of "class" were of utmost importance for survival.
While the play is just a representation of real life, Wilde knew all too well that things could get just as ridiculous and fake in everyday English society as he presented them in his plays. It is sometimes hard to tell with Wilde if, in his plays, art imitates life, or life imitates art.