There are some people who simply don’t like MM. It could have to do with the way he makes out a lineup and/or how he uses the bullpen, but there are some who simply don’t care for him as a person. It could be because of his perceived lack of personality in interviews, his religion, or other traits. Some of these things are hard to quantify.
The facts IMHO opinion are that the Cards (after going to the playoffs 4 years in a row under MM) went to the last day of the season in 2016 with a chance to make the playoffs, even with Matt Holliday (their leader by example) going down. They went to the last week of the season in 2017 with a chance to make the playoffs even after the back of the bullpen issues with Oh and Rosenthal. And this year they are 1 game out of a wild card slot right now, even with the injuries, a jury rigged lineup, and the pathetic performances of Fowler, Wong, and Pham (over the last 2 months).
I would look at Mo rather than MM if a finger is to be pointed. A manager can only play the Cards (so to speak) that he’s dealt.