Not sure. Have they done DVOA for all Superbowl winners? Jets maybe but probably not since there were a lot fewer teams. Colts had a pretty bad defense back 10 years or so. Maybe the Giants in 2007. Their defense came together only in the playoffs if I remember right. Seem to remember Washington having a porous season. Was that 83 or 87? One of Denver's wins was with a bad defense. I would have to do a search.
Here's the DVOA for the last 5 years of teams that have made the Super Bowl:
2017NE - Offense #1 ... Defense #31 ... ST #3 ... Overall #6
PHI - Offense #8 ... Defense #5 ... ST #16 ... Overall #5
2016NE - Offense #2 ... Defense #16 ... ST #7 ... Overall #1
ATL - Offense #1 ... Defense #27 ... ST #8 ... Overall #3
2015DEN - Offense #25 ... Defense #1 ... ST #14 ... Overall #8
CAR - Offense #8 ... Defense #2 ... ST #23 ... Overall #4
2014NE - Offense #6 ... Defense #11 ... ST #5 ... Overall #4
SEA - Offense #5 ... Defense #1 ... ST #19 ... Overall #1
2013DEN - Offense #1 ... Defense #15 ... ST #21 ... Overall #2
SEA - Offense #7 ... Defense #1 ... ST #5 ... Overall #1
Going back farther, looking just at the overall DVOA:
2012 - BAL #8 , SF #4
2011 - NE #4, NYG #12 ---- (Giants had #7 Offense #20 Defense)
2010 - PIT #3, GB #2
2009 - IND #8, NO #6
2008 - PIT #4, ARI #20 ---- (Cardinals had #10 Offense #21 Defense)
So you've got a couple instances in the last decade of mediocre teams making the Super Bowl -- the 2011 Giants and the 2008 Cardinals.
The Patriots through Week 14 this season:
#10 overall, #6 Offense, #21 Defense, #19 ST
The offense is surely trending down after yesterday, and the run defense has to be trending down as well. The special teams may be also.
Overall I would say you're right that occasionally bad teams make the Super Bowl. Once in a while, if they have some kind of weird curse against their opponent (as with the Giants and Pats), they even win.
But the overwhelming trend is of Super Bowl contenders having either an elite defense or elite offense, and often both.