Disagree with him or not, it is essential that the Fed be seen as independent. Otherwise you run the risk of eroding faith in the financial system. And again, even though Trump was only reportedly demanding this in private still shows he has no clue as to what he is doing. It's like demanding that a fire be put out with gasoline.
Some damage may already be done, since the President's ability to remove the Chair has now entered the public sphere ("can he fire him since the Fed is under the umbrella of the Executive?"). More uncertainty when the market is seeking stability.
I know you're a supporter, but surely you must concede that this was not a smart move.
EDIT: And if this leak was floated at Trump's direction as a trial balloon, that takes this up a notch further on the irresponsibility scale.