The Peterman decision was foolish, but other than that, although some of the let-goes have been hard to understand emotionally, he has made this a truly competitive team.
I would add, his biggest overall leadership mistake has been the hire of Dennison. A real test of his ability to lead and make difficult decisions to make the organization better will be what he does with Dennison. It should be an early fire--no matter how or when the season ends--with a goal of hiring an OC that understands football in 2018+ .... Dennison is the OC version of Rex Ryan, had a good system that worked well at a particular point in time, but not now, not for the players and situation you have. And, unfortunate, neither was/is able to adapt.
At any rate, MeDermott's handling of this will be his first big test of visionary leadership.