Although what happens to Antigone in Sophocles' play is certainly tragic (she commits suicide as a result of an unjust decree) and she is considered a heroine in the modern sense of the word (i.e., we admire her because she stands up for what she believes is right in the face of adversity), I'm not sure Antigone is what Aristotle had in mind in the Poetics when he tried to define the hero of a tragedy.
For Aristotle (Poetics 13), the hero of a tragedy was a noble male, who was neither "eminently good and just, yet whose misfortune is brought about not by vice or depravity, but by some error or frailty" (S.H. Butcher translation). This description fits the character of Creon rather than Antigone.
Creon is nobleman whose misfortune is caused not because he is a wicked and vile human being, but because he makes an error in judgement that he does not decide to rectify until it is too late. His failure to reverse course on the decision to execute Antigone, results not only in Antigone's death, but also in the deaths of Haemon and Creon's wife.
We also need to keep in mind that our modern definition of hero is different from the ancient definition of hero. In modern times, we think of heroes as people that we admire and would wish to be like. In literary terms, however, the hero can be the focal point of a story without being someone that we would admire or want to be like.
So, while Antigone is certainly an admirable and heroic person in the modern sense, she may not be the sort of tragic hero that Aristotle had in mind in the Poetics. On the other hand, surely no one would want to be like Creon, but he probably fits the bill for the literary definition of a tragic hero.