A tragic hero is defined by several factors, he must have a character flaw that prevents him from seeing the truth. There is a reversal of fortune due to his mistaken judgement. He usually loses his life at the end of the work of literature, having had a tremendous realization about how wrong he was in his judgement, but it is too late to change anything. His death is usually seen as a necessary result of his own tragic mistakes, sacrificed for his sins so to speak.
Yes Joe Keller is a tragic hero. He fits this description in all ways. He is not an ordinary man, he is a man who has worked his way up from the bottom to be a business owner. His accomplishments make him extraordinary, but he has a blind spot in his judgement. He suffers from a sense of self-righteous pride which prevents him from seeing right from wrong. He makes a huge mistake which results in a tragic occurrence which results in his downfall.
Joe Keller makes a decision to ship faulty airplane parts to the military which results in the death of several pilots and his own son. When he realizes that he killed Larry, that he caused his suicide, his self-awareness is raised and he cannot bear to live with his guilt. He dies as a result of his tragic mistake, his character flaw, his refusal to recognize the bare truth.