Well, you can look at it one of two ways. We were ranked 12th and 2nd in takeaways in 13' and 14', respectively. Then in '15 we dropped to last in the league. So either '13 and '14 were outliers and the D showed their "true colors" last year, or '15 was an outlier and we'll we should bounce back to a respectable number in '16.
If you look at stats like yards per play, points per possession, etc., you see that this defense actually performed marginally better in 2015 than it did in 2013 and 2014. BUT...2015 was still a defensive disaster due to the lack of turnovers.
The difference was the putrid performance by the Cowboys' offense. When opposing teams aren't having to play from behind, they're not taking unnecessary risks, which contributes to turnovers. Therefore, IMO, 2015 WAS the outlier, but nit necessarily for reasons you can pin solely on the defense.