I am having problems with the underlying theme associating appearance to Mathilde. What is the connection?
The underlying theme associated with Mme. Loisel's appearance is that pride can lead to one's fall. Mme. Loisel's greatest sense of pride was that she was born beautiful and her sense of shame came from the fact that she was born to what she believed was a lowly station in life. She felt that she belongedto the rich and famous, so she dewlt day in and day about on her life. When her husband finally agrees to give her his saved money for a gown and she borrows a necklace from her friend she feels that she is beautiful enough to attend the ball and she has the time of her life. All the women want to be her and all the men want to be with her because of her appearance. It is her pride in her appearance that night and the fact that she doesn't have the courage to tell her rich friend Mme. Forrestier that she has lost the necklace. Her pride ultimately paved her way into a life of utter poverty. Her station in life looked fantastic after she began to live her life in poverty and she eventually lost her looks trying to pay back her insurmountable debt.
Mathilde's finery is superficial and it is indicitive of her own superficial psyche. She goes to incredible lengths to be in the "right" social settings, wear the "right" things, but she is really accepted nowhere and the tiara that causes so much fuss turns out to be "paste" that is, a clever fake, just like she is.