I think you can definitely blame the head coach for the overall team performance and makeup. He doesn't make every little decision, sure, but he's in charge of the people who make those decisions. He doesn't make the plays, but he picks the players that do. And so on.
When the overall team performs the same (conservative, reactive, uninspired) year after year, then you can legitimately point to the head coach as part of the problem. Especially when basically everyone around the head coach (players and assistant coaches) has changed in the time that coach has been around. I think there's maybe 3 people who've been around as long as Garrett. Jerry, Witten, and Ladouceur. Obviously, Witten and Ladouceur are non-factors in overall team performance. And say what you will about Jerry, but he doesn't make the gameplans, call the plays, coach the players--nor does he manage the people who do those things etc. That's Garrett's job. If it's not, then what is his job?
Time will tell whether this year is different. But would anyone be surprised to see another somewhat successful season followed by an early exit in the playoffs when matched up against a superior gameplan?