It's much easier to DI out of your standard Jiggly combo than it is to DI out of your, say, standard Falcon combo. Even compared to other characters that rely on drills, she has it worse because of her puny hitboxes.
Almost every character can box out Jiggly because she has tiny hiboxes, no projectile, and terrible movement speed. You can't play Jiggly offensively or defensively. The only thing you can do is wait for your opponent to give you an opening.
Jiggly has a long recovery, but it's very easy to edgeguard. Projectiles eat away at her jumps, she can't recover from below, she's floaty so there's plenty of time to read her movement, and her low priority means she can't fight her way back.
If you look at Jiggly's matchups, the only thing she has going for her are a counter on Ness and a slight counter on Yoshi and possibly Kirby and Link. She's more or less even against Falcon and DK and sucks against everyone else.
Link has awesome projectiles and great range. You can play keep-away with half the cast, especially against someone slow and awkward like Jiggly. His combos aren't shabby either. The only thing lacking is his recovery, but then look at Ness and Fox and Falcon.
I ranked Fox higher than Kirby because he can actually finish his combos with kills, he has a great projectile, and he's faster. Kirby has good priority, but so does Fox. Also, imagine a defensive Kirby against Fox. Fox has lasers, a great shield-pressure game, and speed.