Meh, this matchup is pretty close to even, though kind of strange.
Both characters can kill each other well, both excel at comboing each other, and each has an easy time gimping each other (Kirby isn't that hard for Mario to gimp. If we have to use our UpB, we should be dead (if we try to sweetspot, we get edgehogged, and otherwise we get caped/FLUDDed. We can gimp you pretty easily too though.).
You guys have the upper hand on approaching, survival and projectiles, but we combo slightly better (due to us having a lighter weight), and generally outrange you (everything on the ground minus smashes, especially Fsmash, and I think Bair beats your aerials if well spaced.). Also, our 'Get off me' move, Nair, is pretty bad, whereas you guys have UpB and Nair.
We both have good ledge game and offstage game as well.
Finally, due to our lack of air speed and distance covered per jump, I have found, on both sides of this matchup, that Kirby is one of the easier characters to land a Fair spike on, especially if he doesn't see it coming.
If anyone wants to play this matchup, I have internet and can give anyone a bit of matchup experience via wifail.
Not necessarily. It only gets hard when the opponents Nair is a combo breaker like Ivy's or Yoshi's. Unless Kirby's Nair is capable of that (or something else), comboing him shouldn't be particularly difficult.
He's no Fox or ROB who're ripe for teh ****, but racking up damage should still be pretty much like everyone else. I know it's always been that way for me.
Meh, it's not much harder, but slightly. Generally speaking, Kirby combos Mario better than Mario combos Kirby, but not by much. Both characters are combo whores.
Our Nair can be a combo breaker, but it takes 10 frames to come out if I remember correctly, so it's a biot on the slow side.