Exploding Pineapple is the first upgrade of Path 2 for the Monkey Ace in BTD6. It allows the Monkey Ace to drop pineapple bombs every three seconds that explode after three seconds. Each pineapple has 40 pierce with a small blast radius (approximately the range of an Ice Monkey).
It costs $170 on Easy, $200 on Medium, $215 on Hard, and $240 on Impoppable.
This upgrade is usually useless outside of Race Events, and even then it doesn't pop very much, although every little extra pop does count towards obtaining the Bomber Ace much quicker on Races.
Pineapples can provide extra pops for the Monkey Aces every once in a while, but this upgrade should not be the main source of lead-popping power, due to the slow and random nature of pineapple deployment.