80-20?
That's really, really, REAAALLLLLYYYYYY extreme.
Jigglypuff stays in the air constantly, and like Wario, you're probably not going to be getting many trips in. However, your bananas are still a projectile and they can still be used for momentum. Dribbling around to avoid Jigglypuff and upthrowing bananas is really good in this matchup, as are short hop and full hop banana throws. You can do things like throw a banana closer range at Jigglypuff and bait an airdodge or movement, and follow up with an aerial.
Jigglypuff is probably going to be more in your face without the fear of banana tripping. Her Pound has great priority and chips off a shield surprisingly well. Her fair does nice damage, has decent range, but overall is probably going to be used to kill or gimp you. Her bair is spammable, and nair hitbox lasts deceptively longer than you'd think. Dair combos to rest, and uair...is uair.
Her aerial mobility is definitely better than yours. On stage, you'll have to be worrying about pounds, bairs, fairs at lower percents (to keep it fresh for the kill), and nairs. If pound hits you, it sends you upwards, leaving you open for a uair. Her pound can also be angled up for horizontal survival, or down to surprise you and hit you if you're standing on the ground.
MFK and Monkey Flip in general are good in this matchup, mobility wise and priority wise. Your fair outranges everything she has, I forget if it gets outpriotizied by Pound, but other than that, you don't have to worry about it.
She can catch your bananas; her nair is a good aerial for doing so. However, she can't use them against you. She doesn't have a glide-toss, and an item cancelled throw is pointless because of her horrible ground speed and ground game in general. If she's able to trip you, it'll probably be followed up with pound or fair given the distance away you'd be. You're much better at getting your bananas back, though. A rocket-barrel cancel is worth trying at lower percents.
If she tries to use rollout, just jump and downthrow your banana. It doesn't have to hit her; she'll trip if she rolls over it.
Be afraid of her offstage. Wall of Pain (a combination of bairs, fairs, nairs) can carry you offstage and kill you if you don't DI. Recover like you would against MK; monkey flip if you're high above the stage, and charge rocket barrels if you're a flip's distance away from the ledge. She has faster aerial mobility than MK, so watch out. You don't want to recover close to the ledge, or you're screwed.
Her primary kill move is going to be fair in this matchup, however it often stales out, meaning you won't die for a long percent unless you get gimped or WoP'd. Dair to rest works for her, but try to DI out of the dair as best as possible to avoid that (and hopefully make her whiff her rest). Our primary kill moves will probably be fair and Fsmash (she most likely isn't going to be close enough to the ground for a Dsmash to hit her). Both of those moves may stale out also, so be wary. However, Jigglypuff is much lighter than Diddy, so you'll most probably be killing earlier.
I don't want to put numbers on this, but it's a close MU.