The significance of the necklace being made of false gems does suggest that society places more importance on the superficial, than on what really matters.
That the necklace was a beautiful piece of jewelry is undeniable; it was beautiful enough to catch the attention of not one, but two women: Mdme. Forestier and Mdme. Loisel.
The ecstasy that Madame Loisel feels demonstrates her fixation with the superficial, rather than the appreciation that she should feel for the fact that her husband was good enough to be invited to an important dance.
..her heart throbbed with an immoderate desire. Her hands trembled as she took it. She fastened it round her throat, outside her high-necked waist, and was lost in ecstasy at her reflection in the mirror.
The facetious way in which society interprets what is important is obvious in the importance given to a fake necklace that, in the end, ends up costing her superficiality ten years of her life.
Mathilde is opssesed with being noticed, and have other people be jealous of her. and when she went to Mme. Forster she chose that necklace because it was isolated and in its own box and the box was fancy and she was fooled by its appearance although in reality it is a fake necklace. Mathilde cared for appearances and the necklace was fake but because it appeared to be "WOW" she wanted it.
as for the irony it is because Mathilde wanted that "fake" necklace and thought it was what will upgrade her appearance yet it is what led her to poverty which is ironic!