I dont mean feeding a monster with high RCV to Ammy. Certain monsters will drop with a + next to their egg, and when they hatch will have one of either boosted HP, attack, or recovery. You can check which stat is boosted on the info screen. Fusing a + monster will transfer its bonuses over. The maximum stat bonus is 99.
Since Amaterasu Ookami's leader skill heals you every turn based on her RCV stat, you want to feed her as much +RCV fodder as possible to maximize how much she heals you for.
Similarly, you want to try to feed a monster like Neptune or Echidna as much +ATK as possible, since their skills deal damage based on their ATK stats.
Ammy's diet is probably the closest thing you get to 'messing up'; Siren the Enchantress's Rondo of Healing also depends on her RCV but nobody really cares if you mess up her diet.
Just to be clear, you're saying that each stat can be +99 and not that the grand total of + is 99, right ?
I mean I can have +99 +99 +99 and I'm not limited to say +0+49 +50
Silly question but still extremely worth knowing the answer for. I won't add the stat bonus in the same way depending on the answer.