I have always loved this kind of question. Most players seem to think only the top 1% are "good"players. But honestly those are exceptional players. A good player is harder to define. Some will say its if their damage is good, other will say if they can do mechanics.
IMO, a good player understands their job, has a decent grasp on their combos/priorities with abilities, and can do mechanics. Good damage will come, for example, if you do the other.