Mathilde Loisel was a beautiful young woman who felt she always belonged in a higher class than she was in. She was "trapped" in her class because her parents did not have the money to provide an adequate dowry that would allow her to move into a higher class, and so therefore she hired a clerk who worked in an office. In truth, the Loisels weren't exactly poor. They had a servant, they lived in a fairly large house, but it wasn't how Madame Loisel dreamed her life would be. Madame Loisel was tortured the fact that she lived an ordinary life, and even during dinner she dreams of how her life should be. This is a criticism of materialism and the class system because Madame Loisel didn't appreciate what she DID have. Her loving husband even manages to get an invitation for a prestigious ball, and this only upsets her more since she obviously doesn't have the clothes or jewelry to attend. This trapped feeling of misery will be her downfall throughout the story.
Maupassant makes it clear that Mathilde comes from a relatively poor family but happens to be an exceptionally beautiful girl who could have made a much better marriage if she had had better opportunities. She married a humble office worker who could not provide the pleasures and luxuries she saw other young women enjoying. She was not able to attend the kinds of social functions she could only read about in the newspapers. She was unable to show off her beauty, grace, and charm because of their lack of money and inferior social position. Her life was confined largely to cooking and housework. She felt herself getting older and losing the glow of her youthful beauty. Her husband was a virtual nobody without any great future in the government because he had no important family connections and did not seem to be especially talented or ambitious.