Makes zero sense
If all civilian leaders are poor leaders, how would you expect them to lead Marines, let alone anyone else for that matter? That's kind of a dumb statement. I will say that not every military veteran is better than civilians, but there are foundations that they receive that can lead them to be successful.
One thing that I do like about civilian life is that I get to decide who my boss is. If someone new comes in that I don't like, I don't have to stay there. I can just go find a new job. I chose my current position mostly because of who my boss was going to be.