Yoshi's always been good. Just a strange sort of good.
In 64, he was tank + priority **** + combos out his @$$ + amazing recovery + shield destroyer + projectile ***** (I want 64 eggs back NOW). In 64, he was good because everyone was good (except Link, and even then, he wasn't that bad...maybe).
In Melee, he was tank + combos yeayuhz! + heavy mindgamer + gimper + various other utility roles (very good CC, aerial CC, great projectile, move versatility). In Melee, he was good because he was balanced (which few characters were) and because he could 0-death most of his "bad" matchups with relative ease after he got the first hit in. He had amazing shield tricks and could fend off most attacks with enough tech skill. He was "bad" in Melee because 1] Sheik CG'd him up until 2006, 2] his true tech skill hadn't been revealed until maybe late 2005, 3] he could be easily camped, 4] nobody good in the United States used him in tournament as anything but a joke character, and 5] because balance in Melee meant you were a bad character (and before anyone argues, this is just true, so don't even bother).
In Brawl, his roles have completely changed. I think the only real constant he's had going into Brawl is that he's still a combo machine. He edgeguards pretty well, but overall, his survivability has been crushed because of the new weight scaling (I've only been able to pull off early- and mid-200% stocks). Is he still good? I think he is. He's get versatility to an extent and all of his moves are really good. The only true move nerfs I think he got were to up tilt and forward smash. Of course, with Snake and Meta Knight, everyone's definition of "good" becomes skewed--and rightfully so. My definitions would skew if all I dealt with were moves that were, in the true sense of the phrase, too good.
Anyway, to answer your questio--er, I guess you never had a question.
Hm.
I guess all I'm saying is Yoshi's good, always has been, just has never had anything broken that he could abuse, so he was "bad" in the sense that most mor--er, people, look at the game.