Are Most Trainer Overrated to Some Extent?

They make a small difference in the overall picture. However those small differences can be the difference between winning and losing. Good trainers don't say much to a fighter during the fight, that's not helpful. A good trainer is calm and clear, and keeps instructions as simple and concise as possible. The coaches you see trying to give a lecture to fighters in between rounds are the guys who are doing more harm than good to their fighter, they also tend to be the 'egotistical' coaches.
I also agree with everything Saintpat said, good post.