Does Madame Loisel deserve the punishment she received?

Whether Mathilde deserves it or not, there's certainly an element of poetic justice to what happens to her. Madame Loisel always thought of herself as better than her humdrum existence. Deep down she always felt herself to be a princess, someone who belonged in high society. That's why she's so keen to wear the necklace—it's a ticket to a better life, a life of elite social functions in which she gets to look beautiful and all eyes are upon her.
But the necklace, like Mathilde's social aspirations, is completely fake. And because she needs to borrow huge sums to pay for a replacement when it gets lost, she ends up living in dire poverty. Mathilde wanted to be a lady of quality, but now she's a common drudge, struggling to make ends meet. That is her punishment. Whether she truly deserved it is largely a matter of opinion.