Yoshi Is by no means TERRIBLE, but he's quite a long ways from GREAT.
the problem with yoshi is 4 things
1. lack of amazing players (lets be honest im sure we're all good and decent, but none of us are out placing first in tournaments which im sure IS possible with yoshi)
2. Whatever yoshi has theres another BETTER character that has the same thing yoshi does but well.......even better :/
Power? Donkey kong
Air priority and air speed? Wario
Combos? metaknight
3. Yoshi's shield game, lets be honest yoshi's shield while STRONG, doesn't make up for the fact that he can't jump out of it, if yoshi has to shield your flow is broken and then you're put on the defensive, and yoshi as a defensive character is terrible
.
4. Lack of good matchups against higher mid/high tier characters. so many high tier and upper mid characters give yoshi trouble wolf, falco, pikachu, R.O.B, D3, SNAKE!!!! I STAB AT THEE!!!!, diddy kong, its tough to compete with that. not saying it's impossible to win if anything yoshi needs to wait and be patient so he can punish in these matchups. but its hard and this hurts how he places in tiers.
personally i find yoshi's saving grace though is his surprisingly decent matchups with some of the upper/top characters.
Olimar is a good matchup
Metaknight, i know is bad, but i cant tell you how many people come up to me telling me hoe envious they are of yoshis grab release on meta.
Wario doesn't fair too well either
not to mention yoshi doesn't have much of a problem with most low tier characters (except for link which i PERSONALLY don't consider a bad matchup but thats for another time)
Looking at these though I wouldn't put yoshi ANYWHERE above Mid-Mid. yoshi is a very balanced character, has cool tricks, a decent everything (except shield ><) good matchups vs low, and a few good matchups against higher characters, but thats just me