Antigone can be considered a tragic figure for several reasons. It is best to start with a definition of a tragic hero. According to Aristotle's work, Poetics, a tragic hero is one who is virtuous and good, but one who experiences misfortune due to some error or weakness. Based on this definition, Antigone perfectly fits the role of a tragic figure.
First, she is very virtuous and good. She wants to honor her brother's death (Polynices) by burying him. This is especially important, because one the worst things that can happen in the Greek world is not to have a proper burial. Second, she is also brave, because she is willing to risk her life in burying her brother. If you recall, Creon the ruler of Thebes expresses forbid this at the pains of death.
As the story progresses, she is detected and she is awaiting death by starvation in a cave. The whole episode evoke great pity and sympathy among the readers and the audience. What makes this even worse is that she kills herself. Had she waited a little, she would have been set free. Tiresias the wise blind seer persuaded Creon to stop his madness.
As for her flaw, it is in her unyielding nature. In fact, her name, Antigone, means something similar. In Greek the "anti" can mean against. So, her tragic flaw is that she will bury her brother no matter what the cost.
In conclusion, Antigone is a tragic figure.