Does Creon believe that an effective tragic hero must have elements of evil and good?

No, Creon does not believe that an effective tragic hero must have elements of evil and good. In fact, he argues that the best tragic heroes are those who are wholly good, and that their downfall is caused by their own hubris. He cites the example of Oedipus, who was a good man but was brought down by his own arrogance and pride.