The fact that the necklace is a fake may or may not have some kind of moral meaning. You could take it to mean that wealth, or appearances more broadly, are false. Against the backdrop of wealth and appearance, we have the contrast of Mathilde's poverty. Being poverty stricken may ruin her appearance, but it forces her to become responsible and hard working, and perhaps makes her appreciate what she had before. You could take away a moral such as, wealth just keeps you wanting more until you ruin yourself, while poverty teaches appreciation.