Mathilde's lifetime of daydreaming most definitely turns out to be very harmful indeed, not just for her, but for her long-suffering husband.
Despite coming from a long line of civil servants, Mathilde has got it into her head that she actually has noble blood coursing through her veins. This makes her feel that she's so much better than she actually is, that she deserves a much higher standard of living than her husband's meagre salary can afford.
Instead of accepting her relatively modest lifestyle—which is still a good deal better than most people would've enjoyed in France at that time—Mathilde is thoroughly dissatisfied, always believing that she's born to the kind of life led by her fictitious aristocratic ancestors.
As Mathilde cannot live in the present, she must live in an imagined future, a fantasy in which she is a lady of quality admired and adored by the cream of high society. That's why she jumps at the chance to attend the Education Ministry ball; with her beauty, poise, and elegance, she will be the center of attention, dazzling all those lucky enough to lay eyes on her.
But before she can go to the ball, Mathilde must have some beautiful jewels to wear. Otherwise, she'll feel somewhat underdressed. And so she makes the fateful decision to borrow the eponymous necklace from Madame Forestier, which will eventually lead to her ruin and that of her husband.