That was considered at one point, but there was a huge outcry amongst monsters. It turns out they are all very concerned that players will make them look ugly on purpose.Quote from ImmortalSol
So yeah, if it's about customization you should be able to customize the monsters looks as well.
I mean it's one thing to allow players to change their own appearance, but a whole other thing to allow them to change someone elses appearance. That wouldn't be fair at all.