I think as long as it's just a cosmetic touch that has no impact on gameplay and doesn't have much to it, there will be the least amount of complaints from both sides. Of course, I'm thinking along the lines of, "You get a pet, it follows you around and appears with you in other people's plazas, but that's it. You can't interact with them or dress them up."
I'm not against the idea of pet interaction or dressing (as long as it doesn't go to virtual pet levels), but considering Splatoon, with a side interest in customization, is focused to be a shooter, spending so much development time on more pet functions beyond my thought on the idea would not only take plenty of time away from the game's focuses on battles and customization, but would also bring up complaints from those not as much into the idea of pets, including not only people who understandably don't want to see so much development time put into something so minor, but also the unjustly stubborn people who will complain, "Splatoon is a shooter, nothing else, more customization in any way sux, go play Sims dummy." I hate such people too, but spending too much time on this wouldn't make many people happy.