Merged with our general MU hub.
I think what you'll want to do is learn Yoshi's weaknesses in general and see how you can abuse them with Kirby.
In general, Yoshi struggles with
- Fast, aggressive shield pressure (Yoshi has poor OOS options)
- Characters that wall him out with good hitboxes and frame data (think Sheik).
- Characters that he has difficulty actually hitting/comboing. This is compounded by a lack of reliable kill setups.
- Characters with really good movement in Neutral, forcing commitments and generally being hard to fight back against (Yoshi lacks good, noncommittal options here).
As a Kirby player, I think your best bet would be to try to make something of weaknesses 3 and 4 here. Unless I'm mistaken, Kirby lacks the amazing frame data and hitboxes to really apply 1 and 2. However, his multiple jumps and small stature contribute well to points 3 and 4.
I'm not sure off hand, but Kirby might be capable of ducking under Yoshi's grab. This can really limit options in Neutral, especially because Kirby is small, making it hard to hit a grounded Kirby with aerials. Kirby also has multiple jumps, which you can use to bait out responses (say we're trying to punish your landing, you can fake us out then punish our whiffed attempt) and then punish. The multiple jumps would also help you avoid combos/juggles, and they definitely help you recover, effectively removing a lot of our main damage sources. This also makes it harder to kill you.
On the flip side, Kirby's horizontal movement is kinda slow, so being airborne is pretty risky if we get under you just because of our ability to shark you. Ideally you'd never want to be way high above us in the air. In neutral, you'd probably wanna hang out within a certain cone of safety that Yoshi usually struggles to deal with.
If you'll examine my amazing screen capped Youtube video plus MS Paint diagram, you'll get an idea of what I'm talking about. Basically, if you're hanging around in the air between those orange lines (Roughly speaking) you're safe from an immediate threat from Yoshi. If you're closer to Yoshi you'll probably want to be higher than the lines suggest, but this is just to get an idea. Basically, within this regions you can threaten Yoshi without really being too threatened in response. This, coupled with your multiple jumps, can force Yoshi into shield or to take some action, which hopefully you'll be able to punish.
As far as edge guarding goes, try not to chase Yoshi off stage. Kirby lacks the air speed to really threaten Yoshi out here, and if Yoshi passes you while you're out for the gimp you'll be getting edge guarded and you'll be sad. Yoshi can have a hard time getting up from the ledge, so try to force him there, and then punish his attempts to get up.
I don't think the MU is easy for Kirby, because Yoshi is pretty fast and has a nice projectile, but I don't think it's hopeless. There's definitely stuff to do, and I think learning about Yoshi's problem spots is a good first step. If you can identify what Kirby can do to make Yoshi uncomfortable and start abusing it, you'll see more success.