Yeah, grab-release -> u-air on Jigglypuff can be gotten down with practice. I land it basically all the time now. Having a guaranteed kill at 85% is really nice to have (especially since Yoshi tends to live twice as long against Jigglypuff).
There's not a lot that Jigglypuff can do as long as we stay grounded and play defensively. Since Jigglypuff has no good ground approaches, the fact that she has to approach through the air severely limits her options. Her f-air tends to get stale, and since that's her only good kill move we live for a long time.
Rest is a problem sometimes, but it's rather easy to avoid the set-ups for it and avoid doing moves that would let Jigglypuff gain an opening.
Heck, despite her good recovery it's really easy to d-air -> footstool her if she recovers level with the stage. One footstool is usually all that's needed depending on the number of jumps she has. Don't expect Jigglypuff to recover low often, though.
To be honest, I see this match-up to be closer to +3 than +1. There hasn't been a single instance in which I felt Jigglypuff was even a minor threat out of all the ones I've played (and I've surprisingly played a lot). +2 seems fine, though.
I agree to a point, but I honestly do not think it's THAT easy. Since there alot of things to consider.
For starters, Jigglypuff would be almost no different than Sonic in this matchup. Yeah, we can egg camp her and play defensively, (This is good, don't get me wrong
) if Jiggs has the lead, she doesn't NEED to approach at all. Which would then make this match up more difficult because we can't get to her if she's camping/flying around all over the place.
Second, her combo ability isn't exactly terrible either. Jigglypuff can hit Yoshi's blindspot just as easily as Kirby can. If a Jigglypuff can hit you with say like...A fair at about 35% up,
it could potentially mean death if you're not careful enough.
At high percents, you want to stay onstage and out of the air in general. Anywhere above the ground is in the "Jiggly territory". Though, her biggest ground threat moves are her Dash Attack and DACUS Usmash. So just watch out for those moves. The main move that leads to rest is her Dair. When at high percents, AVOID THE DAIR AT ALL COSTS. Going offstage for a gimp Jigglypuff is itself a very risky idea since that's Jiggly's main territory. You screw up even slightly, and your stock can pretty much be declared gone.
Buuuuut, with all that being said, I'd give this MU +1 in Yoshi's favor.