I think you could categorize them as a "great" defense. All time great? 85 Bears, 01 Ravens, 02 Buccaneers? Not necessarily, but there are a lot of different variables at play which could make them appear "good" instead of "great".
1. The Rams offense scored extremely fast. The defense was on the field a lot which is never great, but the defense still only gave up a hair over 15 points per game and only allowed more than 20 points 4 times that season (one in the season finale against the Eagles when starters were resting).
2. The Rams actually had an offense.
With respect to Refrigerator Perry, the 85 Bears, 01 Ravens, and the 03 Buccaneers, their defensive accomplishments stood out all the more because...well the Rams had more offensive talent in 1999 than these three had combined. When the defense HAS to be exceptional in order to win, it makes it all the more impressive. Having said that, the Rams weren't a team that HAD to outscore people. Holding a lot of teams under 14 helps a ton.