Not that great, apparently.
From 1992 - 1995, Troy Aikman had an average (between games) quarterback rating of 94 -- this does not count playoff games. In that period, the Cowboys won 3 Super Bowls and went to an NFC Championship. If we look at consistency, Aikman had a game-to-game variation of 0.29 (standard deviation divided by mean).
In 2011 and 2012, Alex Smith has an average (between games) quarterback rating of 93 -- this does not count playoff games. If we look at consistency, Smith ALSO had a game-to-game variation of 0.29 (standard deviation divided by mean).
The league-wide QB rating is about 4% higher today than it was then, so you can say that Troy Aikman had an inflation-adjusted QB rating of 97.
From 1992 - 1995, Dallas was in the Top 5 in defense and in the Top 5 in rushing every single year. Sound familiar?
So boys and girls, you need a good quarterback to go all the way with a great defense and a great running game. That's it. This idea that Trent Dilfer and Brad Johnson are the only two non-great QBs to ever win a super bowl is simply uninformed.