The mood of "The Necklace" is somewhat wistful, as readers experience Mathilde's desire for all of the things she does not have. There's a certain hopelessness to Mathilde's situation that colors the overall mood of the story, though it's also worth noting that most readers probably recognize the superficiality of Mathilde's materialism. In turn, the story's wistful atmosphere is tempered by the sense that Mathilde is unreasonably preoccupied with appearances and things that ultimately won't bring her happiness.
As the story goes on, however, the pity that readers feel for Mathilde increases, since her life becomes increasingly difficult and sad. As Mathilde loses even the little status she had at the beginning of the story, readers acutely feel her powerlessness over the direction her life has taken. During one of her rare breaks from the rigorous work of maintaining her household, Mathilde muses on other paths she could have taken:
What would have happened if she hadn’t lost the necklace? Who knows? Who knows? How strange life is, how full of changes! How little it takes to doom or save you!
The repetition of "Who knows?" emphasizes the agony Mathilde feels about having seemingly squandered her potential for happiness. Nothing about her life has gone according to plan. Now, though, readers are more likely to pity her—after all, she has undergone quite a bit of misfortune, and though this misfortune is largely due to her greed and materialism, it's easy to feel bad for her. This wasn't necessarily the case at the beginning of the story, when Mathilde seemed unreasonably vain, but the hardship she undergoes ultimately invites pity, shifting the story's mood from somewhat wistful but critical to something a bit more somber and heartfelt.