I once had a boss who always felt you don’t want to prepare too much because as he put it “it takes the edge off”. I think you want the edge to keep your brain engaged in high gear, as long as you have the ability to think on your feet like Trump does (and as Hillary and Obama suck at doing).
Once we caught one of his peers locked in his office with a camcorder, practicing a presentation he was going to make. We surreptitiously pulled the plug ;-) The guy was pissed when he found out but my boss assured him we were doing him a favor!
Oh, my. What does he define as "too much" preparation?
When I am going to present, I have a stack of material that I have read ahead of time. I use that material to prepare a PowerPoint presentation. While some people try to fit everything they want to say on PowerPoint slides, I don't do that. My slides are just talking points which I use to remember what I want to say, and I do not prepare for a presentation by memorizing what to say. Usually, my presentations go off pretty well.