The word mature in the context of the question can mean either that Mathilde has now grown up from being childish and foolish and that she has become more sensible or that she carefully considers the outcome of her actions before committing to them. This sort of maturity implies that she should consider others' reactions to her behavior and assume that their responses would be influenced by what she has said or done, either negatively or positively, in both instances.
One would expect that Mathilde would have learnt a lesson about her foolish desire for material things after her difficult ten-year trial. She does, however, still display her immaturity by blaming Madame Jeanne Forestier for the struggle that she and her husband have had to endure. She responds to Jeanne's comment about her condition by saying:
"Yes, I have had some hard times since I last saw you, and many miseries ... and all because of you!"
The trial that they had undergone was, in fact, not Madame Forestier's fault, but hers. She was the one who had insisted on borrowing the necklace in the first place and her remark is, therefore, inappropriately immature in this regard.
Mathilde's immaturity is also obvious when she tells Madame Forestier about how she had replaced the lost necklace and then...
...smiled with proud and innocent pleasure.
Her action here illustrates that Mathilde's immaturity, in this regard, was borne from a somewhat childish belief that what she had done was something good. Her innocence indicates her lack of in-depth insight and understanding about exactly what price she and her husband had to pay for her foolish desire to impress. They had essentially lost ten years of their lives just because she had to indulge a fantasy. One would expect that she would not still, at the end of it, feel proud of having sacrificed and lost so much.
One would assume, after Madame Forestier's shocking exclamation about the lost necklace's true value, that the realization of how much has actually been lost to replace what was, in fact, a trinket, would hit Mathilde like a ton of bricks.