It depends on the game if the game is more social orientated like roleplay enabling Character Customization for only Premium is unfair. If the game is more like shooting, tycoon, etc orientated where player’s goal is to earn money, kills, points, etc then no it’s not unfair because the game isn’t about looking good.
Also it doesn’t really matter you’re the developer it’s your choice. I understand putting in hours of work for a game and wanting some kind of return. Premium users cash cows lol.