I really like the idea from it, and I see your logic on how it is mostly outclassed by Natural Cure, but I feel like lots of people would still purposely use this ability so that they wouldn't have to switch to deal with poison, as it is with Natural Cure.I'm not sure you'd consider this a flavor ability, but to me this does seem inferior, Immunity, for the main fact that Kerfluffle is related to a pillow, and if it did share a connection with pillows,(other than it's body resembling a pillow) then it would be that had stuffing on the inside of it's pillow-like body. So, since stuffing more of absorbs poison and is not really affected by it, it would make an interesting ability for Kerfluffle.