I'm surprised no one posted about Brinstar yet, simply because every time stages come up in the Kirby boards, you get Brinstar fan-love going.
Anyway, against Mario, I think, of the neutrals, BF and FD would be my top choices, because as great as Mario is a gimping, if Kirby plays it smart, we're more likely to gimp Mario. I personally hate YI, though it screws with my Mario a lot, so I guess that would be good (then again, maybe you guys aren't ******** to accidentally walljump>buffered UpB in the wrong direction... The number of times I've done that while trying to leedge camp...). Also, those ****ing shy guys always appear in front of crucial fireballs (they never interfere when it's regular fireball spam, but if I'm going to fireball gimp, you can bet there'll be one swooping out of nowhere to intercept it.).
Anyway, on subject, RC and Japes are both good for Kirby, and they don't seem great Mario stages (I mean, JJ isn't bad for him, but not quite as good as for Kirby). Pirate Ship is good too, and I don't really see many Mario players getting that water spike off.
I actually wouldn't want to bring you to Brinstar personally, if only because I'm afraid of a Super Jump Punch through the stage and even moreso of that Dsmash/Usmash (I know a lot of you probably think they suck, but personally, I LOVE these moves.). I wouldn't want Dream Land or Corneria either, since getting star KOed by a Utilt would really frustrate me.
Well, that's all I can think of atm, I'll come back to it later.