If the standard for judging a coach were winning, that can be argued to play in his favor. Relative to the rest of the league, Garrett wins at a reasonable rate. His issue has been the up and down seasons. Problem is, people won't credit him for the wins, and blame him for the losses.
Do people really think the quality of coaching is what's see-sawing back and forth in Dallas in recent years? Because the coaching has been pretty consistent. And yet the results have carried wildly. That suggests to me the problems are actually elsewhere, though nobody wants to hear that.
More than anything, people want change because they thing that represents progress. I understand the reaction, and as much as I support him generally, it's hard not to be swayed by that when the blowout pile up. But it's Fools Gold.
Jerry Jones created a situation that makes it very difficult to win in Dallas. But like it always does, it comes down to the players. You build a team around an identity, take a bunch of hits in free agency, whiff on your own signings, and you have a net talent loss. Then you lose 4 all pro players for significant time and your good team becomes an average one. That's how the league is setup to work.