MobileApp vs MobileFrontend?
No and no. As the extension page states, this extension "serves styles and scripts for use in the Wikimedia Mobile Applications"; mobile applications in this context refers to the Android and iOS apps officially supported by Wikimedia's mobile team. This extension allows developers to add CSS and JS for use by the aforementioned mobile apps without having to release new builds of the apps (I'm not sure about Google and Android Market, but at least getting a new version of an app published on Apple's App Store is a pain, or so those with more experience in that particular field say).
Chances are that the extension you're looking for is MobileFrontend and you likely won't need to ever think of installing this extension.
Thanks for the clarification, makes sense.
Ok, now I understand as well. But maybe this explanation should be promoted to the front page because the one-liner is a bit vague?