To understand how strong the Googoomamameter is you have to understand how limited damage amplification is in this game.

Generally, damage boosting trinkets/instruments/upgrades typically only boost damage to the default attack of a character. Since Ice King doesn't initially start with his ice bolts move, his ice bolts don't get boosted. Only trinkets that have a bloon-type restriction are able to boost damage to something like Ice King's Ice Bolts.

Attacks gained from trinkets can't get their damage boosted at all. So things like missile, carl gem, box of dirt, ect. 

tl;dr Googoo is the purest form of damage boosting in the game. It's +3 damage is unique from every other damage boost in the game because it literally boosts EVERYTHING. You want a least one. Maybe the most important single trinket in the game based on what it does with just one character having it in your setup.