Joker Balloons were part of a small challenge to pop a small, tied down (and together) group of balloons with devious smiles on them.
They were a lot like the TYGER Security Cameras, that were mounted objects that you must destroy for the Riddler's Challenges.
Batgirl: A Matter of Family Incident
Multiple patches of Balloons were string around Seagate Amusement Park. Destroying them all, along with Joker Teeth and Harley Quinn's Jack-In-A-Boxes were required for a achievement/trophy.
Arkham City Incident
Small groups of Joker balloons were scattered around Arkham City. They were part of Riddler's Challenges and encompassed bits of the Industrial District and Amusement Mile.
The Joker Balloons in Arkham City
- The balloons could be popped with the Batarang, REC, Freeze Blast or just by passing over them while gliding.
- Balloons also appeared on Knight, but were purely decoration. They could be popped just like Joker's balloons.