To master Yoshi you need to Master DR and DT, which are hard to due unless you were like a top melee player or have enough time or are focued enough to learn it.
You also need yoshi's CG's and some of them are quite hard, Also you need to pretty much change the way you would recover.
For a lot of chars it's DJ, Use a special.
Depending on your special whatever happens.
For Yoshi that 2nd jump is crucial, you can use it if your a risk taker, but to play it safe you pretty much always keep it in reserve.
To utilize Egg Toss for recovery....you've got to be top notch with Aerial Influence, otherwise Yoshi will basically just hover in the air... and not grab the ledge...I remember being so close to the ledge at one point egg toss'd and forgot to keep the AI, and I started dropping, I tried to save myself, but I didn't AI properly, I was too quick to Egg Toss again. Yoshi then died,
Well O.K maybe Yoshi isn't number 1. But he's pretty hard to master.
Some people will detest me for it....but MK isn't as hard, Im not a master MK, but he's quite easy to pick up, and many of his tech's aren't exceedingly hard. the IDC can be hard...to an extent, but basically for MK it's all about basics, knowing your spacing, getting your timing down, thats a master MK. Which is fine, but I don't feel he takes a huge amount of devotion to "master" him.