Jigglypuff had only been notable for a funny singing prank in the Pokemon anime. I guess that's the reason why she made the roster of Super Smash Bros. since the original. Like all the games before, I've never been a big fan of Jigglypuff.
Her moves are very imbalanced. While her singing powers and that super-strong special are very powerful, it's a miracle to actually hit a real opponent with one. She often moves like Kirby but without any grace or the skill that he has.
Speaking of Kirby, he can do the rollout move when he takes Jigglypuff's power. It's a decent move when used right.
Her Final Smash allows her to grow to a large size. It's not very powerful and only really is useful on small stages.
I think I only won once online with Jigglypuff. I can't even recall facing a player using her either.