Ok, so here's the thing based on what I can give you:
1) You should use Shiek or Rosa + Luma against any confirmed Yoshi player. Maybe Villager, but I recommend the former.
2) Yoshi IS a strong character, but For Glory scrubs are bad players. Here's what you should look for:
-Punish EVERY Yoshi Bomb with a DACUS (dash attack canceled u-smash). Every. Single. One. Time it so you don't get hit by the star. They're gonna do it quite a few times from up high. Keep punishing it until they stop. They also might return to the ground with d-air. Just avoid being directly below him after he's had breathing room to time a bomb or a d-air. You really shouldn't be using d-air like that, but some Yoshi players do because they love that 33%. You can pressure from below so long as you have a good u-air or he hasn't started one of those two moves.
-Respect him when he's below you. This is for any char, but him and a few others especially. Rosalina has at least some options in her d-air, but in general, you want to get to the ground safe as possible. He can't rack up u-air damage if you move even a small bit to the sides, so throw off his aim and get moving.
-He can eat you. If he's camping while you're at 100+ percent, he's probably trying to land a scrubby Egg Lay. If you get eaten, focus on returning to the stage BEFORE focusing on breaking free unless it's clear that you're going to (not) fall. It's possible for them to air chase and then eat you while you're recovering, but the only person I've seen put that into practice is myself, so I'm not sure if that's something to worry about.
-Most scrubby Yoshi players don't play into his entire kit. They either over-rely on d-air, Yoshi Bomb, or whatever else. Use these holes in their game plan to your advantage by exploiting the weaknesses of the move they over-rely on. This depends on the guy in question, but Yoshi's moves all have strong properties...and a weird, glaring flaw. It's hard to explain, but long story short is that if you crutch on one aspect of this character, Yoshi won't carry you that far at all. He's strong when you use a good deal of his kit's strengths in tandem while covering his openings and weak spots with his mobility (another strength of his).
-Lastly: Be wary of his f-air while recovering. It's the fastest f-air meteor smash out of the 3 (DK, Mario, n' Yoshi), and it can hit through small projectiles with pro timing. The meteor effect always occurs so long as the hit-box hits you from above, and a good Yoshi's gonna try to line the hit up and land it rather quickly. Not sure about bad ones, though.