This is a great question. In a sense, I suppose we could argue that both Nora and Torvald could be classed as tragic heroes. In the final Act in particular both of them are granted moments of insight that allow them to see themselves and their marriage for what it really is. Consider the following quote from Nora:
I have been performing tricks for you, Torvald. That’s how I’ve survived. You wanted it like that. You and Papa have done me a great wrong. It’s because of you I’ve made nothing of my life.
Nora sees herself very clearly for perhaps the first time in the play, and she forces Torvald to see himself clearly and their marriage in a similar way. Both realise that their tragedy revolves around a personal flaw or failing. For Nora, this flaw has been her propensity to have her world shaped by the men around her. For Torvald, it has been his habit of babying and patronising his wife, and treating her as less of a human and more of an inanimate object, such as a doll. In this sense, both suffer from hamartia that helps trigger their final downfall that is represented by Nora's exiting the doll's house that has entrapped her for so long.