Statistically, Yoshi should be a beast. He's heavyweight, decently agile, and has high damage % on many of his moves. Unfortunately, Yoshi's downsides are things the metagame has been designed to exploit.
Despite this, I feel the following makes Yoshi a very viable character.
A well designed game, the the tools to play it: Fact, Yoshi does not gimp people easily (save D-air, but thats a perk, not a goal), so Yoshi wishes to get the opponent's damage high then attempt some sort of knock out. Fortunately, many of Yoshi's attacks set up for eachother. Yoshi's up-smash can be combo'd with itself at low percents, allowing Yoshi 30-50% on unwarry opponents, his eggs have just enough knockback/stun to set up for another well aimed egg, and Yoshi is even packing a chaingrab. I personally don't attempt most of my non d-air kills until ~130%, and when the opponent starts seeing those numbers, he will likely panic when you try to flip the killswitch. Fortunately for Yoshi, he packs the mobility to turn panic to suicide. Yoshi is reasonabally quick on the ground (above average, borderline fast), but is also virtually unavoidable in the air. Yoshi packs multiple fall speeds, and a fast fall finisher, making landing that much more of a chore.
Yoshi packs 2 of the best disruptors in the game: Jab and N-Air are two of Yoshi's quickest moves, and pack more then enough range to punish before punished. If you want to trade fisticuffs with Yoshi, by all means. Jab pings with both Snake and Ike's quickest attacks, and is much faster, often resulting in you landing the blow. N-air kicks often into Egg Toss range, can stage spike, and can even be used to Wall'o'Pain with experience. Enemies off the stage start taking damage much more quickly then on the ground, especially when D-air becomes a threat or connects.
Yoshi can recover offensively: No secret, Yoshi can do a raising N-Air, B-Air, U-Air, even D-Air for those who wanna be nutty. Yoshi can jump into egg lay, oftentimes reversing edge guarding if used correctly. Yoshi can super armor certain attacks while rising, he can even air dodge through the entire thing. To top it off, Yoshi's Egg Toss guards the very area he moves too when he does it. Worse comes to worse, Yoshi has two suicides and 2 spikes (though 1 of each is the same) to punish any misattempted gibs, Egg lay suicides, F-air spikes, and D-air does both.
I may continue my rant later.
