Other interpretations of "The Necklace" exist than the one your question suggests. Mathilde is a normal human being who longs for a finer, more artistic existence. She is a woman with imagination, and she is smart enough to know that society is not all it could be. Your question blames Mathilde, but evidence exists that society is at fault here as much or more than Mathilde.
Maupassant is considered a naturalist writer. His writing is about human victims of forces they cannot control. Mathilde is not ideal, but she is a normal, intelligent human. She's smart enough not to just accept her place in society. She's creative and imaginative and has as much right as anyone else to the things she dreams of. She is similar to a frustrated artist, who longs to create and is not allowed or able to do so.
Just to demonstrate this with one point. Only a person too poor to have any experience at all with jewelry would not even consider the possibility that the necklace may have been costume or fake jewelry. Anybody with any experience with jewelry would know that that was a possibility. But Mathilde does not. Why? Not because she's stupid--she's not. She doesn't consider the possibility because she is not of that world, she has no experience with jewelry. The situation is beyond her control, beyond her capability.
Again, Mathilde is in part a victim of societal forces beyond her control.