The point of the Contest is that it's Diana's catalyst. Because of the Contest, she decides to leave her home, her family, Paradise itself, in order to protect mankind from evil, and bring the Amazon way to the outside world via example. The contestants are masked ( at least in most versions) so no one knows who is whom.
It's less about Diana, her abilities, and the perceived " fairness" by those outside of comics reality ( us).
It's the motivator for Diana. She has to make that choice. To betray her mother's wishes. To test herself. To find that strength in herself to meet her destiny .