Punish missed aerials, be careful of late hit boxes
Good spacing will punish hard
Try to parry rest (nice try Jiggs mains, just shield rest unless you want max style points) or counter it.
Is really light, you can take a stock off her earlier most of the time. Also shield-breakers are able to instantly kill her as well.
Sing and Rest are really easy to punish once blocked or whiffed.
Her rollout tends to be pretty predictable at times, so countering it would be an optimal choice.
Watch out for the sing hitbox. it's bigger than it seems. It even hits under platforms, if the Jiggs is singing on the platform.
Beat out her aerials due to poor priority
Easy to edge guard
Bad airdodge
Easy to edgeguard
Poor ground game
Bad grabs and throws
Pound (sideB) is actually a disjointed hitbox, so it will win Jiggs a number of trades. Furthermore, it does significant shield damage (only taking 2-3 to break a shield). Luckily though, it is very punishable. If your shield is strong, block it and punish. If your shield is weak, stay evasive until the shield recovers.