In this story, Mathilde becomes obsessed with her appearance after she and her husband are invited to an elegant party. She spends a large sum of money on a dress, and then she borrows a diamond necklace from a friend. This proves to be their ruin. On the way home from the party she loses the necklace, and rather than suffer the disgrace of admitting this to her friend, her husband spends all the money they have and takes out loans so they can buy a replacement. After that they live in poverty, working hard for 10 years to pay back their debts. In the end, Mathilde finds out that the necklace she had borrowed was an imitation, worth only a fraction of what she had paid to replace it.
Certainly, Mathilde's materialistic attitude is responsible for their downfall. However, even before de Maupassant tells of the party and its aftermath, there is a short introduction that makes Mathilde's materialism clear. The author writes that she is unhappy with her simple lifestyle because she feels that she was "born for every delicacy and luxury." She is dissatisfied with "the poorness of her house," and she imagines more elaborate and luxurious surroundings. She laments that she has "no clothes, no jewels, nothing":
And these were the only things she loved; she felt that she was made for them.
These are some of the quotes from the description of Mathilde's character at the beginning that you could use in your paragraph. After she finds out about the party, of course, her materialistic attitude becomes starkly evident. She proclaims that she has nothing to wear, although her husband points out that she has a nice dress that she wears to the theater. She demands a large sum of money with which to buy a new dress. Even after she buys a new dress with the money her husband was planning to use to buy a new gun, she whines that she is "utterly miserable at not having any jewels, not a single stone, to wear." She insists that "there's nothing so humiliating as looking poor in the middle of a lot of rich women."
We see, then, that de Maupassant gives many indications throughout the story that emphasize how materialistic Mathilde is.