I think my biggest advice is to stay very mobile, and poke with ftilt. Unlike other matchups, your ooS options are more limited vs Jiggs, so be mindful when you shield.
Moving toward Jiggs with an aerial with intent to hit her at any point, at all, will get you bair train'd. Thanks for playin.
But after playing HBox, I can say cheerfully that I don't think the matchup's our worst matchup, nor do I think Jiggs advantage is huge.
Disrupting her spacing with missiles is key, I think. She can aerial through them, but at that point, she's not considering the space as much and you can use that opportunity to your advantage.
Keep in mind the rhythm that the Jiggs uses to aerial. Bairing the air clears space and pressures, but it has a distinct rhythm that you can use to get in and poke for an opening.
In general, just try to play gayer than the Jiggs. Forcing your way usually leads to getting *****.
Do not attack Jiggs shield, and be careful of the bair train.