How is one coach "better" than another? Well, this is sports so the answer is easy. They win more games and championships than other coaches.
We can't compare McCarthy to other Cowboy Head Coaches, that wouldn't be fair. McCarthy has been a Cowboy HC for a year. Now, if you just want to compare first years then Barry Switzer is the best HC in Cowboys history, Tom Landry is the worse.
Obviously proper context has to be established and the conditions have to be examined. Some HC's inherit a rebuilding project, other come into a ready made contender.
Some think the HC should be the hardest working guy in the whole franchise. However, being a HC involves a great deal of management and that includes who you hire to be your assistants. management is getting things done through others.
Jim Lee Howell was the Giants HC when they won the NFL championship in 1956. He wasn't known as the hardest working coach in the business.......Would you be if your offensive coordinator was Vince Lombardi and your defensive coordinator was Tom Landry?
Howell was a HC with an abundance of, shall we say, "discretionary" time?
Maybe McCarthy isn't the best field general but the guy knows how to utilize his coaching assets.