The Importance of Being Earnest has many themes, which are the author's (or the upper class') viewpoint on different aspects of the society's values.
The first value that is satirized is duty and respectability. These were crucial to the aristocratic class of the Victorian society. Earnestness is the determined and serious attitude to do the right thing, but in this play it is used to describe a man who tries to deceive other people. Appearance was everything to the Victorians, and it was all right to do whatever you wanted if you appeared to do what was right.
Popular culture was another theme that is satirized. The mindset that nothing good comes out of France is used in this story. For example, Jack said that his brother Ernest died in France.
Secret lives were also used in Victorian times because of that culture's repressing norms. While appearing to lead morally upright lives, Jack and Algernon, in this story devise ways to do whatever they want by pretending to visit other people who are either sick or get into trouble so that they can have a pretense to do whatever they want.
These are only three of the most important themes, but there are others in this play, like the absence of compassion, the theme of religion, marriage, etc.