Energy Ball Jellicent isn't completely stupid. Outclassed, probably. Specs Jelli was a thing at one point, and Energy Ball would allow it to hit the most commonly used Pokemon (Rotom W). Now if it was Absorb Jellicent, that would be completely stupid (since it gets both Energy Ball and Giga drain, which far outclass it); or if it were a physical set with Waterfall and Return or something...
4 SpA Jellicent Energy Ball vs. 252 HP / 252+ SpD Rotom-W: 78-94 (25.6 - 30.9%) -- 99% chance to 4HKO after Stealth Rock and Leftovers recovery
Standart Jellicent vs Standart Rotom-W
252+ SpA Choice Specs Jellicent Energy Ball vs. 252 HP / 252+ SpD Rotom-W: 168-198 (55.2 - 65.1%) -- guaranteed 2HKO after Leftovers recovery
Okay, its legitime...
but ONLY on specs sets.