Nora's desire to tear up a costume symbolizes her growing discomfort with her secrecy and duplicity. She desires to be unburdened of her secret, but worries about Torvald's response when he finds out.
Nora has been living with a secret for quite a while before the party is held that takes place toward the end of the play. She has been finding ways to pay Krogstad for the loan she took from to save Torvald's life years ago.
Because Nora lied when she borrowed money from Krogstad, she must continue lying to repay the money.
By the time in the play when the party is to be held, Nora has been threatened and confronted by Krogstad. He has threatened to expose her secret loan (and the forgery that secured that loan).
Nora is under a great deal of pressure as she prepares for the party and gets her outfit ready. At the party she will be called upon to perform a dance, in costume, and this poignant irony is almost overwhelming. She has already been performing, playing the part of the meek and helpless wife when she is actually quite effectual and even manipulative.
The desire to tear the masquerade clothes into pieces suggests the degree of pressure she feels in continuing to maintain her secret and her performance. This desire symbolizes Nora's desire to be free of her secret, to become herself and stop playing the role of helpless wife.
Also, Nora wants to be freed of Krogstad's threats and find a way to save herself from legal action and public humiliation. Desiring to tear up the costume is also a desire to rid herself of all the pressures she is under.