Do you symapthize with M. Loisel? Why or why not?
Are you referring to Monsieur Loisel or Madame Loisel? If you mean the former, then definitely - how can the reader fail to feel sympathy for someone who is married to such an inappropriate wife? She is plainly disatisfied with her position in life and is unable to accept her poisition in society, instead choosing to inflict her desires and envy on her poor husband, who works very hard and makes sacrifices so she can have what she wants.
If, however, you are intending your question to apply to Madame Loisel, that makes it a very interesting debate. Certainly, at first, I feel that Maupassant deliberately makes her out to be unsympathetic, for the reasons given above. Although we can relate to her plight, she deliberately dwells in a fantasy world and makes no effort to embrace her reality:
When dining at the round table, covered for the third day with the same cloth, opposite her husband, who would raise the cover of the soup tureen, declaring delightedly, "Ah! a good stew! There's nothing I like better...", she would dream of fashionable dinner parties, of gleaming silverware, of tapestries making the walls alive with characters out of history and strange birds in a fairlyland forest...
One can't help feeling pity for the husband, especially when he tries to help his wife by securing an invitation for them to the evening reception, to be only met by scorn and tears.
However, by the end of the story, I personally feel far more sympathetic for Madame Loisel, as she meets the challenges of poverty head on and takes responsibility for paying off the debt:
She played her part, however, with sudden heroism. That frightful debt had to be paid. She would pay it. She dismissed her maid; they rented a garret under the eaves.
This response, somewhat unexpected, shows a real resilience of character and an ability to take ownership for our actions. It also serves to make the ending that much more darkly humorous and, in some ways, rather tragic.