Interesting point. Personally I wouldn't mind it if it were like you said and simply a cosmetic change. In Yoshi's Island DS, Baby Mario doesn't do much anyway.
You seem to be a big fan - how would you go about buffing his moveset? I'm one of those guys who love Yoshi but can't stand playing as him in SSB.
Just one thing that Yoshi really needs is simply that his moves need more 'oomph'. His smashes for one aren't as satisfying as those of others to connect with. Mostly because they simply lack knockback power; that always felt like one of Yoshi's major drawbacks for me, personally. What he also needs, is for his tongue grab(s) to actually be as long as they physically appear. I believe that in Brawl, his tongue didn't have the reach it should have had. Also, his tongue just needs to be a liiittle bit longer; like, make it at least as long as Samus' grapple move.
And then there's Yoshi's egg related problems. I'm sure you know what I'm talking about. Yoshi should NOT go helpless after using it; it's barely got ANY use in the air, and it's just way too easy to roll off stage if you have poor control or have too much lag in the match (when playing online). It doesn't need additional horizantal distance when used in the air, just so long as it does NOT put him in helpless state. And then of course there's his problem of being unable to jump from his shield, which is one of his biggest problems. He clearly JUMPS out of his egg in his stage entrance animation; why can't he jump from it by manual input? Also, I didn't like how his eggs from the Egg Toss/Throw move were less manouverable. And make his one Meteor Smash easier to hit with, too; it's too easy to fail in Brawl.
That's about all I can think of at the moment. I'm certainly not as competitive as other people around here and mainly just play for fun, but those are what I feel should be the least of the changes on Yoshi's side.
Also, Baby Mario did make Yoshi run faster (somehow) in Yoshi's Island DS, and enabled Yoshi to hit certain blocks that wouldn't appear with any of the other babies. So he definitely WAS helpful there.