Hmm, I find wavedashing towards your opponent very effective with yoshi. Probably follow it up with a down tilt a and then a shffld fair. The f-tilt is a good follow up to dair.
I don't know about you guys, but I like Yoshi's roll much more than anyone else's. I think he/she's invulnerable like 99% of the time in that roll, so you can actually roll away from repetitive a's. Don't know about the stats for it though.
Oh yeah, one more thing. Always light shield with Yoshi... You'll see why. Excellent retreat; it'll probably confuse your opponent.