Ive been thinking about it, and I speculate it might be an useful idea to split up the battle pet rating into two seperate ratings.
Namely one for pve and one for pvp.
Chrominius is a strong example of how a pet can be strong in one field, but weak in the other. Very potent against npc trainers, but fairly weak against a competent player as they just will swap out of the howl debuff.
And some abilities are not terribly useful in pve, but strong in pvp. Such as force-swaps, as pve petbattling encourages a decoder ring style of fighting mobs, so you bring the pets the specific tamer's pets are weak against. So you can rarely if ever benefit from upsetting the order in which you are destroying them. Stuns and other CC abilities, which strong in pvp, also can disrupt a pve battleplan as it will force the npc to swap.
You could go further and have a pve and a pvp top 20 as well.