The game have already a good variety of game play with types of monsters, moves, runes & relics. Concentrate on another way to enrich the game play is a nice intention, but sometimes just add a little something cool & fun can be productive.

So, it can be very interesting to offer the possibility of giving back the baby and juvenile skin to monsters (paying it with gems) without altering monster's moves & characteristics.

I agree, some skins just look cuter/cooler than the mature ones. Like Gastosquish, he is so cute as a baby but meh as a mature.