What might have been the quality of Mme Loisel life if she had not lost the necklace? Is her life better or worse now?

This is a very interesting question! Through text evidence, inference, and supposition, readers can decide what the quality of Mademoiselle Loisel's life would have been had she not lost her friend's necklace. Yet the question of whether she is better or worse off requires us to delve into the depths of her desire and is more difficult to answer.
Guy de Maupassant's short story entitled "The Necklace" shows the protagonist, Mathilde Loisel, desperately longing to be wealthy and admired. However, her station in life does not allow this. She is not from a notable family, she has no money or position, and therefore she settles for marriage with a lowly clerk in the ministry of education. Despite her reality, she longs to satisfy her cravings for the finer things in life. Consider the following passage:

She suffered endlessly, feeling herself born for every delicacy and luxury. She suffered from the poorness of her house, from its mean walls, worn chairs, and ugly curtains. All these things, of which other women of her class would not even have been aware, tormented and insulted her. The sight of the little Breton girl who came to do the work in her little house aroused heartbroken regrets and hopeless dreams in her mind. She imagined silent antechambers, heavy with Oriental tapestries, lit by torches in lofty bronze sockets, with two tall footmen in knee-breeches sleeping in large arm-chairs, overcome by the heavy warmth of the stove. She imagined vast saloons hung with antique silks, exquisite pieces of furniture supporting priceless ornaments, and small, charming, perfumed rooms, created just for little parties of intimate friends, men who were famous and sought after, whose homage roused every other woman's envious longings.

Because of these longings and her husband's great desire to make her happy, he secures an invitation to a ball for them. She is able to buy a nice dress but fears she is not appropriately adorned. She borrows a beautiful diamond necklace from her childhood friend Mademoiselle Forestier. At the ball, she loses the necklace. Because of her pride, she is unwilling to tell Mademoiselle Forestier the truth. She and her husband end up buying a new necklace to replace the lost one. They have to borrow the money and end up losing their modest lifestyle. For ten years, they work to pay the debt. Mathilde ages greatly in that time period.
From a strictly monetary position, Mathilde would have been better off if she hadn't lost the necklace. Although she was dissatisfied, she lived a comfortable lifestyle. She had a maid to help with housework. She had a comfortable home. She had enough money to buy a dress worth 400 francs—a healthy sum for a piece of clothing. She and her husband had savings from an inheritance.
It is my assertion that the quality of Mathilde's inner life would have been no better if she hadn't lost the necklace. She was already a bitter woman, lamenting the fact that she couldn't have the fine things she desired and felt she was entitled to.

She had a rich friend, an old school friend whom she refused to visit, because she suffered so keenly when she returned home. She would weep whole days, with grief, regret, despair, and misery.

She was so entrenched in her covetousness that she wouldn't even maintain or nurture a relationship with an old friend, because it brought her too much heartache to see her friend enjoying all the luxuries wealth provides. Dissatisfaction with one's life brings about its own misery, no matter the presence or lack of material comforts.
It wasn't truly the loss of the necklace that ruined Mathilde's life. It was her consuming desire for what she could not have. If she could have made the choice to be satisfied with her modest yet comfortable life, she would not have come to such financial ruin. If she had learned to be satisfied with her devoted husband who tried hard to please her, she might not have become bitter and dissatisfied with her lot in life.
Had she maintained a closer friendship with Mademoiselle Forestier and not been so bound by pride, she might have been able to tell her the truth about the necklace disappearing. If Mathilde had told her the truth, she would have known that the necklace was an imitation, and not worth anything. Her life savings and ten years of her life were given to repay something that was of no value. Mathilde misses the things in life that are of true worth, like the selfless giving her husband shows, and the simple pleasures of love.
Mathilde is a static character, and other than outward appearances she shows no changes throughout the course of the story. The one thing that changes is that she has the memory of being beautiful and admired at a ball once upon a time. She was able to pretend to be what she longed to be on that one night. But it is a shallow desire that can never bring her true happiness. The point that she failed to grasp is that life's joy is not found in material things, which glitter and shine but have no lasting worth. True joy is found in relationships and love for one another. She disdained her husband due to his lowly station and refused to be satisfied by his selfless love. She even failed to nurture her relationship with a friend due to her jealousy and pride. In the end, even her illusion of being what she always desired to be is shattered when she finds out the necklace was a fake.