I've been thinking about this too much lately and I feel like I can't coexist with the Yoshi boards with my feelings, but Yoshi is the worst character in the game.
I don't say this without excessive study of gameplay and data. I've learned a lot about this game and I can say that Yoshi is fundamentally flawed and doesn't have the tools to compete against the top characters at the highest level.
1. His shield radius is poor. I've learned a lot about shield radius playing all these months. Every character besides Yoshi has a shield that is quite a bit larger than Yoshi's while full. This allows them to block attacks/projectiles inches sooner than Yoshi. Large characters like Bowser and DK benefit from their excessive shield radius...Yoshi has an introverted shield. He's taller than his shield, so his head is especially vulnerable.
2. Frame 16 Fair, Frame 11 Bair. Besides Dedede and Shulk, Yoshi has the slowest combination of side-airs in the game. Every character besides Yoshi has either a faster Fair, faster Bair, or faster Fair and Bair than Yoshi's Bair, which is his fastest side-air. Every character has at least one better air attack to spam than Yoshi. Yoshi depends on Nair to get quick hits, but Nair requires that Yoshi put himself into vulnerable situations (overlapping hurtboxes).
3. Bad grab, bad grab game. Not much more to be said than that. At least ZZS can lock you into a grab and 0-death you. Yoshi's grab game depends entirely on the opponent screwing up their landings or air-dodging at the wrong time. It's a slow grab with poor tether range and a weak throw - truly the worst grab in the game.
4. He's really hard to play against, but the matchup is still in the opponent's favor. There are many things Yoshi can do to you, but for the most part they can all be played around. Yoshi, as a character is more or less a barometer for whether or not you are good at the game or not. If you play fundamentally well and can react to his unusual movement, Yoshi is very easy to predict and punish.
5. Overall, bad frame data. Besides Jab, F-tilt, N-air, and U-air, Yoshi has mediocre to bottom ten frame data. Frame data compares poorly to just about every match-up in key areas, especially side-airs. Yoshi has more endlag than most characters (except he doesn't have a jab combo, which is where most characters suffer from lag, and where most scrubs perceive Yoshi as 'laggless'). I'm not going to ramble on about the bad comparisons, but you have to look at the numbers, analyse them to understand them.
Yoshi has a lot of unique positives in his favor, but he also has unique flaws and some of his positive qualities work against him (his high double jump makes it impossible to 'feint' away and retaliate).
I love Yoshi, Yoshi stands tall in this game, but he isn't viable. He's overrated by the community, often hated by the community, but he's just a fictional dinosaur frog horse who isn't viable in this game against good players.
Anyways, I figured I'd say this here because no one reads the Yoshi threads because people don't like Yoshi anyways I'll retire from posting now for sure so Yoshi thread can go back to normal (and toil away trying to improve a character who is destined for bottom tier five years from now unless the community never improves).