The language of the story is a combination of simple and highbrow, establishing Mathilde’s pretentiousness.
The premise of the story is that Mathilde was born into a family of clerks, but felt she should have been born into a wealthy family. Therefore when she only was able to marry a clerk, she was very unhappy. This desire to be a higher station than she is seems to be demonstrated in the way language is used.
There are many passages with simple language, such as this one.
She dressed plainly because she could not dress well, but she was unhappy as if she had really fallen from a higher station….
There are also sprinklings of more sophisticated words and ideas like this one.
Mathilde suffered ceaselessly, feeling herself born to enjoy all delicacies and all luxuries. She was distressed at the poverty of her dwelling, at the bareness of the walls, at the shabby chairs, the ugliness of the curtains.
This specific language use can demonstrate pretentiousness, because she wants to be more than she is. You should keep in mind though, that some of the differences are due to translation since the story was originally written in French.