However, in my opinion it would add immersion and depth, and a hint of progression. But I understand that the game focuses progressing more in relationships. You definitely do have a point there, might be too much work for the artists, probably would be actually.
We don't agree on player customization as part of game depth. In the majority of cases for games, cosmetics (skins) are fluff. Even in Pokémon X and Y where alternate costumes, eye color and skin ones were introduced, they're nothing more than fluff. The game will still be 100% intact and fully functional without losing its core points without the customization. At most, player customization tweaks how players connect with the avatar or playable character, and that's only if the alternate options even appeal to the player to begin with.
Sure, if we have more time on our hands and want to take the time to sit down and specifically code the options to let players change how they look (then also worry about whether the feature can be turned on or off, if it's permanent or the players can swap at will, and a plethora of other concerns), then great. But we don't have that time nor are we in the position to place such a feature above others that bring more value to the core game.