It's hard for me to judge Garrett because he has the worst handicap in sports. He is very nice, respectful, etc so he can't really do too much about Jerry's mishandling of the roster.
Garrett does need to go back to the drawing board with his offensive scheme but I think he has some of the tools to be a decent HC. No way in hell should his first crack at it been the Dallas Cowboys. Once he's scapegoated by Jerry (and the moron fan base) he should take an assistant job somewhere for a year or two, and then try to be a mid level D1 head coach somewhere like Mississippi State, California, Colorado, Kansas, VIRGINIA, etc.