Always hard to compare because the level of competition is variable.
Statistically, the 2018 pass defense is slightly better than 2012- same yards per game given up and 0.2 TDs per game fewer given up through the air.
The rushing defense in 2018 has given up 27 yards per game more than the 2012 team. The big difference is that 2018 team has given up roughly 1 rushing TD per game more than the 2012 team. Thus the difference through 12 games in points allowed- 207 in 2018 vs. 124 in 2012
Since it takes offense and defense, the net positive yards total offense per game (yards gained vs yards given up) in 2018 was +17 yards per game vs 2012. The point differential for ND vs opponents in 2018 is virtually the same as through 12 games in 2012, +16.5 ppg 2018 vs +16.4 ppg 2012.
I don't think the 2018 ND team overall takes a back seat to the 2012 team. I hope they get to play Alabama in the playoffs and since everyone seems to say this is the best Alabama team ever under Saban it will be interesting to see if ND really does sustain another 28 point loss.