Speedy characters in brawl are a pain by design. In any open air game, speed tends to pwn power.
Meta does have respectable range, but on the fact that he lacks a projectile, he has a weakness to grabs anyways. You have a choice here, take it in the air, which his f-air and b-air are pretty nasty, or keep it on the ground. By taking your fight to the air, you both take away a lot of his speed (metaknight is slow in the air like all floaters) and put him in a bad spot for all of his specials. You b-air can catch him good, but he can smack through it if he times it right.
Being on the ground has its advantages. Metaknight is quite slow to recover from all his specials except for shuttle loop (his flip in the air where he starts gliding) if he lands it correctly. Most people have issues with mach tornado because it tends to shield stab (hits an exposed part of the body causing the damage to register on the player), but as Yoshi's shield always covers his body, Yoshi's worst shield in game now becomes a very effective counter due to its one strength. Bottom line, shield his specials. If you can though, he's open above himself in the tornado, so with a bit of balls, you can shove a d-air right down it (especially if he rises off the ground).
Metaknights are fast, but it comes with a price, they often have their moved planned out ahead so they can execute them with split second accuracy. Get him off his game and you might see some holes develop as he rethinks his attack. He does have things he can do between plans, but when you make even a half second gap, it gets to them, because they know they coulda landed at least 3 hits in that time, and would be very succeptable to yours. Don't rush this match, metaknight users aren't the most absolutely patient bunch. Get him into the 90+'s with suttle nudges and grabs, they pop him right out the game. Be warned, finish him in that shot, if metaknights onscreen, he can come back.