Mathilde is really not meant to cause much pity if we go by the direct characterization provided by Maupassant:
She suffered endlessly, feeling herself born for every delicacy and luxury. She suffered from the poorness of her house, from its mean walls, worn chairs, and ugly curtains. All these things, of which other women of her class would not even have been aware, tormented and insulted her.
This particular piece of information makes the reader wonder what exactly is there to feel for Mathilde. It is clear that she lives in genteel simplicity, but she has enough to live by. She is married, has a home, and we learn that she even has a parlor maid which she obviously may not even need. She doesn't like this poor girl, either. Mathilde treats her badly.
When Maupassant says that her surroundings "insulted her" early on in the story, it shows evidence that Mathilde is not a likable character in the first place; that she only has a sense of entitlement, and not a clear understanding of what her reality is, nor what appreciation looks like. This is the big irony. We would think, as readers, that she would inspire candor and sympathy. The author is adamant that he will present us with many character traits that will make us shy away from that.
In the end, we hate to feel catharsis over the way that she finds out that the necklace that she loses, and wastes her life away trying to pay back, is made of mere paste and fake gems. It goes to show that class cannot be measured by money or social standing. However, we also sort of root for her when she finally gets her dream of shining brightly at the party. In fact, there is an element of empathy that surfaces when we think that her happiness was so short-lived. It happens to the best and worst of us. That is, perhaps, the one aspect of her humanity that is the most evident in her character.
Therefore, the combination of different emotions may actually numb us to Mathilde, making us realize that life is just not fair and one just simply cannot go around expecting to have everything go our way.