If She Stoops to Conquer features any kind of moral lesson, then it is that it is important to be genuine in relationships. Much of the story revolves around the gulf between appearance and reality. The Hardcastle house is mistaken for an inn. Mr. Hardcastle is mistaken for an innkeeper, and Kate is mistaken for a barmaid. Ironically, these errors help reveal the true character of Kate's suitor, Charles Marlow, a young man who appears impudent and rakish in some scenarios and cripplingly timid in others. Initially, these qualities form an inconsistent picture of Marlow and hinder his relationship with Kate.
Marlow appears to be shy, but in reality, his charm and intelligence are repressed by his fear of rejection. He is unable to be himself around women of his own class because he fears being humiliated by them, while he is more at ease among working-class girls who do not hold the same amount of social power. When he thinks Kate is a barmaid, he is able to be himself and allow their romance to blossom. By the end of the play, Kate knows Marlow's true self and Marlow learns that his beloved barmaid is high-born. Now that he knows she can accept him as he is, they can happily wed.
While it is easy to write the story off as frivolous farce, the moral rings clear: relationships between people who are comfortable being themselves are most likely to flourish.