She is a hero.
Antigone maintains her loyalty in the face of powerful forces that tempt her to betray or at least let go of that loyalty.
There are some interesting contradictions in Greek tragedy and we see at least one of them in Antigone. We see that love and affection are valued, yet family takes precedence over bonds of love and marriage. Somehow, Antigone's suicide is honorable because it is in keeping with the heirarchy of values which places blood before law, blood before marriage, and blood before love.
Yes, it could be argued that Antigone is indeed the tragic heroine of Sophocles's play. She is of royal birth, she is a sympathetic character, and she meets an unhappy ending. Essentially, the question of whether she is a tragic hero turns on whether you think her pride, stubbornness and loyalty to her brother are flaws. Sophocles seems to have thought of them in that way, and so have many scholars of the play.