The direction of interface messages in MobileFrontent seems to be always based on $wgLanguageCode.
So, for example, if $wgLanguageCode is "en" and you ask for a page with uselang=he, the interface messages will appear as LTR, and that would be wrong. Among other things, the ellipsis at the end of the search label will appear on the right-hand side, even though it's supposed to be on the left-hand side for Hebrew. For comparison, you can easily use http://en.wikipedia.org/?uselang=he and see English content with Hebrew interface.
Since MediaWiki 1.18, core MediaWiki supports good separation between interface language and content language (Bug 6100). The scenario described above is not, by itself, very important, so this bug does not currently have a high priority, but ideally BiDi practices from core MediaWiki should be used as much as possible in MobileFrontend, too.