Can being too good a speaker hurt your career? This is a question i get every several months. I used to think this was a strange question but i reflected on it and figured: of course you could! In the same way that if you show up ten minutes early to work everyday, you may end up alienating someone and so they may conspire to get you fired.
So should you simply make yourself mediocre? I believe and have seen in my experience, that for most people, the better you speak, the further you will advance in your career. You may have other flaws in your skillsets or in your surroundings that may prove to be a hinderance despite your expertise in speaking.
So it is not impossible. But would you want to be in an organization that awards mediocrity and punishes doing things effectively? I would hope not. You are better off being as great a speaker as you possibly can.