The same argument can be made for tattoos. Why ruin the design of perfectly good skin? And not so long ago, it was the common opinion.

Stickers are a way of expression. Sometimes, I don't like the idea being expressed, sometimes I do. Clean is fine too.

Phone covers are usually for protection. And a reason I hate phones with a glass back (thankfully a dying trend). It is fragile, slippery, heavy and not structural, it only looks good in a store because people put on a case afterwards. Pre Galaxy S6, Samsung had the perfect back cover, made of light plastic, openable and replaceable, and in the case of the S5, waterproof. You may find it ugly, but under a protective case, you don't see it. In fact, they even sold cases that replaced the back cover.