There are a number of alternative entrances for the Bizarre Room where you can end up on any of the 6 walls/floor/ceiling. The regular floor is where you first enter the Bizarre Room, but getting onto the ceiling is also necessary to progress through the story.
First make your way to the very back of the Lotus forest and go through the small opening.
This leads into the Tea Party Garden, which also has another wooden door. Go through here and you’ll find yourself entering the Bizarre Room right on the ceiling.