When this upgrade is purchased, the monkey disappears.
This also applies to the Multi Shot upgrade for Spike-o-pult.
In Bloons TD 4 iOS, when this upgrade is unlocked, it is misspelled as "Tripple the pain", instead of "Triple the pain".
The monkey operating the mortar tower disappears, and the tower can still attack, meaning that this upgrade makes the tower mechanical like the Tack Shooter which attacks with no monkey controlling it.
The upgrade button icon shows 3 mortar cannons and no monkey controlling it so the monkey disappears when you purchase it.