I have questions about many things in MediaWiki. In which site should I post my questions?
If your question is about using MediaWiki from the perspective of a user, Web Applications is probably okay.
If your question is about setup, configuration, customization, or other topics of concern to an owner/administrator, it looks like Pro Webmasters would be a good fit.
(Both of those sites have about an equal number of questions with that tag as Super User, so it looks like maybe your questions are okay where they are.)
As always, check the site's Help Center before posting. You can always ask on a site's Meta (or chat) if your question would be on-topic. (Assuming, of course, you have enough Reputation to do so.)