What a fun assignment! Of course, before you can write a poem, you need to consider de Maupassant's theme. There are several to choose from: Materialism, Pride, Class. Materialism is seen in Mathilde's jealousy of Mme. Forestier's wealth and belongings; it is also evident in the joy Mathilde shows when she is such a hit at the party. Pride is seen through Mathilde's unwillingness to allow only a flower to be her decoration, and then again, through her unwillingness to tell her friend that she has lost the necklace. Society's class structure is also a big part of the story with establishing the difficult situation that the Loisels find themselves in, both in trying to fit in at the fancy party and in the wake of their loss of their middle class status.
Once you have decided which theme you want to explore, you can then consider how to create a poem to express it. You could write a piece about Mathilde and her expression of the theme or you could write a more contemporary piece (if that's acceptable to your teacher). You could write a personal poem expressing your experience with materialism, pride, or class.