Jigglypuff is extremely susceptible to attacks from below and d-throw in this MU. So needless to say she will be walling you out with bair. At early percents, you could try and use armor to get around her wall but it's not going to get you much.
Your best tools in this MU are RHUS/waterfall, watergun, crawl attack, and hydrograb.
Punish any slip up in Jigg's positioning (for coming down from high up with no jumps) by reverse hydroplane up-smashing across the stage for the surprise early early kill or with a surprise anti air attack using waterfall (beware she could SDI out and rest punish). These two moves can kill jiggs pretty much any time from mid percents and on, especially with RHUS because it will be charged.
one strat worth trying out is bubble, Grounded bubble has a large hitbox to safely space against jigg's back air, and if it lands and puts jiggs into knockdown you have the potential to wavedash in and punish her getup option with aquajet (or RHUS in) and get an early kill.
Watergun is your way to poke at jigglypuff through her bair walls, remember that when you watergun in the air you get a lot of air speed to drift around. Use this to short hop air control away (you fade backwards) while waterguning jiggs if she tries to punish.
Watergun connecting can often force jiggs to the ground where you can get a grab if you're close and a crawl-attack if you're not close.
Crawl attack can combo into fair as a kill option, and maybe aquajet but that would be crazy hard to hit.
D-throw is pretty much a solid kill on jiggs after 100%, which is why hydrograb is so good.
If you run away from jiggs and bait out walling, you can get in close to her and punish the endlag (even though it's tough) with hydrograb. It actually works really well and tends to keep jiggs from being close to the ground, which puts them higher in the air and can lead to good positioning for kills from below.
Useless moves for squirtle: F-smash, F-throw, U-throw, B-throw, D-tilt, Aerial Bubble
Dangerous moves (low reward) for squirtle: Bair, Dair, Nair, D-smash, F-tilt, Jab, U-tilt, withdraw
Safe moves (high reward): Waterfall, RHUS, Fair, crawl attack, D-throw, Grounded Bubble, U-air, Aquajet, watergun.