Not very keen on Jigglypuff outside of 3DS version, but it seems that she's not extremely different from Brawl's installment. You'll want to master her dair drills and back airs. She has very quick, albeit, short aerials which make it risky to fight grounded combatants frequently. You'll want to drill, grab, or pound shielded opponents. Her pound is a reliable move to use against direct opponents and projectiles, especially those with large hitboxes. Her headbutt should be able to help you cover ground quickly and take opponents by surprise, but don't use it too often.
Last, but not least, master her smash attacks. Usually smash attacks come in two categories: Fast and light, and heavy and slow. Jiggly's are somewhere in the middle/heavy, and don't cover much ground. Her down smash is one of the most difficult, yet rewarding of smash attacks, especially if it pushes the enemy off an edge. Her front smash is most reliable, as it covers the most ground. You can set it up with a well placed nair or drill, or charge it in anticipation of an opponent's attack. Her up smash is probably as useless as the down smash as far as reliability, but just as useful for payoff. Your best bet is to combo into it. It's hard, but not impossible, to charge it as a counter to aerial attacks, though, you'll want to avoid doing this with characters like Link and Kirby with dangerous dairs.
Your best bet is to practice edgeguarding in training mode. Practice the power, direction, and reach of her moves. Kirby, Link, and Fox are good edgeguard training dummies. Practice connecting your moves on smaller characters. The best way to become an expert with Jigglypuff against other players is just to learn how to answer attacks, and learn how to lead enemies into her slower attacks. She can enter well, but that takes patience and consideration. Really learn what moves work against which strategies, don't be afraid to experiment.
