What is your standard for "not very good"?
Unless pro football reference is off the mark, the Pats have 4 games in which they gave up less than 80 yards on the ground. The low is 54 against the Vikings. That inept Vikings team put up 241 yards on the Falcons, 185 against the Rams and 111 against the Packers (no AP in those games either).
Look at the
yards per game rankings: KC (4), Miami (6), Jets (8), Cincy (12) and Vikings (13). While the bad run games against the Jets, the Phins and the Chiefs help those stats, those teams have put up high numbers in other games as well.
The Pats run D had bad showings in the 1st, the 4th and the 7th games of the season. Less than 80 yards does not qualify as getting run on all season. The best run defenses in the NFL give up about 75 ypg.