Maupassant may be critical of Mathilde's outward behavior when he exposes her personality is lackluster: she seems greedy, whiny, and critical of everyone else but herself. However, the author humanizes her character throughout the story to the point that the reader feels pity, and wants some mercy for her at the end. This shows that Maupassant is not attacking nor criticizing Mathilde's poor system of values, but showing her overall ignorance and dissatisfaction with life. Therefore, it is evident that Maupassant rather uses Mathilde to illustrate the little prospects women had in the 19th century when they lacked money, whether they were attractive, or intelligent, worth marrying, or not. The author's message may be summarized in one simple statement: Life offers much but some are meant to get little. Society may not be fair, but sadly, it is what it is.
Notice that Mathilde obviously lacks more than just money.
She would so much have liked to please, to be envied, to be seductive and sought after
She wants attention, perhaps romance, and intellectual challenges. Her wedding was planned, as it often happened during that time period and, as previously stated, she had little prospects. Mathilde grew up without enough exposure to sophisticated persons, places, or things. Case in point, the fact that, out of all the beautiful jewels she could have chosen from, she gravitated toward the most extravagant-looking, yet, less valuable piece.
Moreover, Mathilde's husband is a pretty dull man, which is a trait that is not necessarily mitigated by being kind. Who likes dull people around them, especially when one has no choice about it? Mathilde is literally helpless to change her life. As a result, the woman can only complaint about her situation, while not really knowing what else to do about it.
...she cried all day long, from chagrin, from regret, from despair, and from distress
Not all of these are reflections of a woman-gone-bad. These are the sad realities that 19th century women had to endure throughout their lives. Therefore, Maupassant does not necessarily criticize Mathilde, but the limitations imposed upon women like her, when in reality there is very little that these women can really do to make their lives better.
If Maupassant were harshly criticizing Mathilde, he would have juxtaposed her to a husband who is highly engaged in his marriage, or crazy in love with her. He could have made her into a thief, or into a careless friend who may have done nothing to return the borrowed jewels. There the author would have had a great opportunity to show Mathilde as a very mean woman. Rather, he shows her as a clueless woman, perhaps as a silly woman, but more like a product of her time, of her society, and of her upbringing. The criticism is against those factors, rather than the woman herself.