I believe this MU is in our favor. 55:45.
As
Zarxrax
stated, Kirby crouching can be a bit of a pain for us because it makes grabbing or baby Shadow Balls an inconsistent option to use against him. A Kirby who's able to break through our defenses and start strings with fhtrow isn't a particularly fun time either. Kirby also has a few deceptively fast KO moves in his kit, his smashes (his up smash is one of the strongest in the game) and back air in particular, but they're all short ranged. His back throw kills at the ledge and up throw can kill us particularly early especially on a stage with platforms, so it's very risky to choose somewhere like Battlefield, Dream Land, T&C or Smashville vs him.
Our biggest advantage in this matchup is our better range and overall better mobility, both in the ground and the air. Kirby's air speed is pretty bad and his recovery is slow, predictable and easily challenged. Even if Kirby gets the Shadow Ball copy ability, it won't be much of a deterrent to us because we simply can just reflect it back with Confusion. Kirby usually has to respect us in neutral if he have a fully charged Shadow Ball at hand, and Kirby will usually be trying to do empty hops as a mixup, and going in for SH aerials or grabs when the opportunity is clear.
In the air, nair, fair and uair are are particularly useful against him. Up air covers the entire area above Mewtwo, juggles well, and can kill Kirby decently well thanks to Kirby's rather poor vertical endurance. Just know that when in the air,
DON'T CHALLENGE STONE. I know the move is not commonly used in competitive play, but seriously don't do it. Back air is also good for edgeguarding Kirby's recovery or as a ledge getup mixup since the move passes through the stage and can hit Kirby if he's waiting near the ledge to potentially punish us. If a Kirby is trying to trick you with his landing by making small jumps above the ground and jumping again, utilt and up smash are effective anti-airs due to their range.
Recommended Stages: Final Destination, Duck Hunt, (if possible) Umbra Clock Tower