You make at least 2 faulty assumptions:
1. You assume that the manager makes ALL of the calls. Sometimes a player does something on their own. Sometimes even when the manager makes the call, the player does not execute (as I think has been the case much of the time this season).
2. You assume that if Garner had made the "correct" move, the outcome would have been better. Regardless of what the manager calls for, the players must execute, which they have not done for most of the season. Also, I LOVE that Garner is not afraid to do odd things to find what works. The offense has been underperforming big time, and Garner is trying to find something that works. That often means making unconventional moves, many of which will not work. But until the hitters start producing, there's not a whole lot he can do from the bench.
Personally, I think Garner is a great manager. The only reason he is getting so much criticism is because the team isn't doing well, but it's not because of Garner.