His ultimate is extremely high impact, it just doesn't feel like it is. With 0 AP (Which should be expected on Rammus) at rank 3 Tremors does 195 damage every second for 8 seconds. Rammus is intended to force initiation on the enemy team, meaning you'll often have the opportunity to hit at least a handful of their team with your ultimate.
Let's assume Rammus hits 3 enemies for 5 seconds in an average late-game teamfight.
19535 = 2925 damage
This is incredibly significant - and this isn't assuming the high end of the ability use.
Even if you assume you can only hit 1 character with the ability for 5 seconds it still comes to 975 damage.
The visuals and the requirement to stick for the damage to be meaningful makes his ultimate feel worse than it is, in reality it's an incredibly solid teamfight ult.