We've been about as consistent under Garrett as a team can be. We beat the teams we're better than, and lose to teams that are better than us. The toss-ups have been pretty even in terms of win-some, lose-some. We've been in the top-quartile of the league in terms of wins/losses under Garrett, and the team has consistently gotten better and won more games as a result.
And the playoff games, it's been discussed in many threads, but those games weren't lost because of coaching. They were lost because our defense wasn't able to play better because the talent hasn't been very good.
Some fans don't want to admit it, but that's whatever. The team's gone from bad in a poor cap situation to good in a good one under Garrett. There's no denying it.
As far as the general HC ranking goes, these linear rankings are silly. Every NFL coaching situation is different. Guys have different personnel support, different ownership support, different financial support, play in different divisions that are significantly more difficult or significantly easier. It's not an apples-to-oranges thing. What really matters is "is this coaching staff good enough to win an NFL championship." Looking at Dallas under Garrett, the most you can say right now is "maybe." He's definitely been a good coach for us. The question is how good?