What was Mathilde Loisel's flaw?

Written by Guy de Maupassant in 1884, “The Necklace” is a short story set in nineteenth-century France. The principal characters are Monsieur Loisel and his beautiful wife Mathilde. Monsieur Loisel works as a clerk and is very happy with the middle-class lifestyle that he and his wife have. However, his wife Mathilde wants to be rich and upper class. She believes that this will bring her happiness. Vanity and selfishness are her flaws. She covets the superficial things in life such as property, clothes, and jewelry.

She had neither frocks nor jewels, nothing. And she loved only those things. She felt that she was made for them. She had such a desire to please, to be sought after, to be clever, and courted.

When she is invited to a ball, she borrows a diamond necklace from her rich friend Mrs. Forestier. She exudes happiness at the ball and is said to be the most beautiful and stylish woman there. But, on returning home, Mathilde realizes that she has lost the necklace. So that Mrs. Forestier will never know of his wife’s mistake, Monsieur Loisel borrows money and manages to raise thirty-six thousand francs to buy a replacement. Now Mathilde knows what it is like to be really poor, as they move into small attic rooms, and she has to work very hard doing menial jobs for a living. Their poverty lasts for ten years until the debt has been paid off. By then Mathilde looks prematurely old, tired, and is no longer physically beautiful.
On bumping into her old friend Mrs. Forestier, Mathilde admits the truth about the replaced necklace but is told by her friend that the necklace had been a fake. Mathilde now understands how her vanity and selfishness cost her and her husband ten years of their life. Despite that, Mathilde realizes that she is now happy.

Expectations of this world
And the people in it,
Are surely the sources
Of our greatest misery.
The Uddhava Gita #3
If Mathilde Loisel had a tragic flaw, it was the same flaw that most of us suffer from. She wanted to be admired by other people. She had her own natural charm and beauty, but that was insufficient. She needed a new dress for the ball, and once she had the dress she needed some jewelry. After she borrowed the necklace from her wealthy friend, she felt satisfied with her appearance and she was a great success at the ball. She got the admiration of all the men and the envy of all the women, which were just what she wanted. But she had to pay a terrible price for an illusion.