In the beginning of de Maupassant's tale, Madame Loisel is stunning. Although she had to scrounge to find just the right accessories fpr the party, when she makes her appearance,
She was prettier than anyone else, stylish, graceful, smiling and wild with joy. All the men saw her and asked her name, sought to be introduced. All the important administrators stood in line to waltz with her.
Psychologically, we see that she is already something of a mess. Her desire to be in the circles of the elite is overwhelming.
By the end of the story, the pressure of trying to fit into the social circles to which she has no purchase has taken its toll (along, of course, with the trauma of the lost "gems").
Mrs. Loisel looked old now. She had become the strong, hard, and rude woman of poor households. Her hair was unkempt, with uneven skirts and rough re hands, she spoke loudly, washed floors with large buckets of water....sometimes, when her husband was at work, she sat at the window, and she dreamed of that evening so long ago, of that party, when she had been so beautiful and admired.
As you can see, Mathilde's mental state declines in steady measure with her fading beauty.