I mean you only need to not let Marth space correctly (?) I mean, I know avoiding tippers is hard, but it can be done.
which attacks from Ike and Shulk outprioritize Jiggs? I'm almost certain dash attack beats most of their fast attacks(?)
And yeah, you're right about shieldbreak.
And tbh, meh, I've faced some good Marths and some good Shulks but when I lost, I didn't lose because of the matchup but rather mistakes from my part
If both players play perfectly, which is what a MU should be based off of, Jiggly has very few options to approach Marth and Lucina, if they just keep you away with their sword, since their sword can hit Jiggly without trading.
It's not about avoiding tippers, its about simply, not being able to get in easily, and it being dangerous to do so. At least, against melee fighters, like captain Falcon, they can't keep us away very easily because Jiggly's fair has a great amount of priority. It's just...priority doesn't mean much when you're up agaisnt a sword as long as your body.
Also if Jiggly can get in to Ike and Shulk's space, they CAN do some damage, BUT
1. It's dangerous, due to their sword beating out any of Jiggly's aerials easily
2. They both have counter, so you can't just approach recklessly to get inside their space.
only times I've beaten a Shulk or Marth as Jigglypuff were when the players themselves are quite a bit below my level.
It's just not worth the struggle. And that's what makes these bad MUs. They are a struggle. Jiggly has to try so much harder in the MU.