The plot of the story is ironic, but the tone is simple and straightforward. It is told by a narrator who speaks in simple language, which leads the reader to believe this is a simple, straightforward tale of loss and suffering. The tone reflects the superficial mind of the central character, Mathilde Loisel, and the mind of her simple, kind-hearted husband. For example, the self-centered Mathilde thinks the following:
She had no clothes, no jewels, nothing. And these were the only things she loved; she felt that she was made for them.
Mathilde's inability to see beneath the surface of things, mirrored in the story’s unreflecting tone, makes her unable to see, or even imagine, that the glittering diamond necklace that so attracts her might be a cheap fake or that the values it represents might equally be false. Her husband, though a good man, takes Mathilde similarly at face value: he cannot see how shallow she is in the same way that she can't delve beneath her superficial desire for pretty things.
This inability to discern the true worth of things leads the couple to pay full cost for a real diamond necklace when they lose the necklace Mathilde has borrowed. The great irony is that Mathilde finds out in the end that the necklace she sacrificed everything to replace is a worthless piece of paste.
When we refer to tone, we are talking about the attitude a writer takes towards a character or subject in a story. A tone can normally be summaried in one or two adjectives, such as cheerful or optimistic. So, to work out the tone of a particular text, you need to bear in mind such aspects as word choice, the theme and the way that characters and issues are presented.
This story, as with many of Guy de Maupassant's short stories, is focussed on grim irony as characters have to endure shifts of fortune that defy our expectations. The way in which Mathilde is shown to spend so long daydreaming about having wealth and her frustration at her standard of living (which should be entirely acceptable), makes her eventual fate of being consigned to a life of drudgery and hard work ironic in the extreme. There is a massive disparity between her dreams of "great reception halls hung with old silks" and the poverty to which she is reduced at the end of the tale. The fact that the cause for her poverty was actually erroneous only serves to confirm the ironical tone of the tale.
We think the author wanted us to feel dumbfounded, because of the irony at the end of the story.