There are a total of five pirate lookouts. The trophy is bugged or glitched in that you might not need to use all of them in order for it to pop.
Candy Kingdom
-- This is revealed shortly after interrogating Admiral Candy Corn on the right side of the walkway.
Evil Forest
-- After interrogating Peppermint Butler, there will be a route highlighted by Marceline. Further in is a Trigger Action for Jake that leads up to this lookout.
-- Returning to search for Fern, you'll rescue BMO. Use him to open the gate to the left of the cage he was just in (on the walkway) to find this lookout and open the door back to the boat.
Fire Kingdom
-- After passing Cinnamon Bun, turn left and use Jake to get to the higher ground. Immediately turn to your left to find the lookout.
Firebreak Island
-- As you're collecting the cores for Torcho, you'll pass right by this lookout.