Board Thread:Support Requests - Designing Your Wiki/@comment-5193807-20130604200122/@comment-10688548-20130604210332

Uncyclopedia isn't actually using Monobook, but Vector. Vector is the new default MediaWiki skin, which is found on Wikipedia. Monobook is the old default MediaWiki skin. Compare and contrast with the following links:
 * http://en.wikipedia.org/?useskin=monobook
 * http://en.wikipedia.org/?useskin=vector

Uncyclopedia is designed as a parody of Wikipedia, so when they were acquired by Wikia they were permitted (as a special case, and also having the benefit of being a very large wiki) to keep the Vector skin so they could better emulate their more serious counterpart.

There isn't any way for you to change the skin for all users joining your wiki, only for yourself. Special URLs, skins, Wikia-skin extensions tend to be reserved for extremely large special cases like memoryalpha, uncyclopedia, et cetera.