The wiki only has two skins: legacy MonoBook, which is the default one (though customized) and Vector, which is not maintained. As such, Vector isn't even set to show ads.

MediaWiki allows non-users to set skins by using URL parameters. The problem is that it doesn't stick once the URL changes by clicking on an intrawiki link.

Test - http://bulbapedia.bulbagarden.net/wiki/Pikachu_(Pokémon)?useskin=Vector