I think displaying quality of pets would depend heavily on how the armory shows this information. From what I've seen, the armory shows both wild pet quality as well as pet item quality. Example: Darkmoon Rabbit is shown as epic on the armory (even though it shows up as rare in the Pet Journal). On the other hand a captured rare wild cat will show up as rare.
Since WcP would need to rely on the armory to keep collection qualities accurate, this mixed information makes it somewhat troublesome. If quality designation were left entirely up to users, who's to say if someone actually has a rare X, Y, Z pet? I know this might not be a concern or apply to everyone, but the more we can do to cut down on false collections, the better it is for the community as a whole imo.
But that's just my two cents. Breanni and Weena will ultimately decide if a quality feature is possible for a future update. ^_^