There are two locations that the Mushroom people can be found. The first cage, right beside where Princess Bubblegum was stored, will have a thorn vine keeping the cage closed. The second is on the top level near where you unlock BMO out of his cage and can find the second pirate lookout. Use BMO to open the gate to get them free. You might want to go in and try to push them around if you do this and the trophy doesn't appear.