I've seen people on servers with very inappropriate skins (nude, disturbing) and I have some reasons why it shouldn't be allowed.

1. It's just plain disgusting to most people.

2. I know for a fact that there are kids that play Minecraft servers that are about from as low as the age 7, and this isn't helping their "clean mind".

3. This is bad for you because if you have one, most people will ban you from their server.

So please, if you see someone on a server with an inappropriate skin, report them. You never know, you could be giving some people relief from never seeing the person on that server ever again.