Sandstorm works decently as a web app on mobile devices. Also, certain apps have mobile app support. For instance, Davros actually can use the ownCloud client. Tiny Tiny RSS has an app embedded in Sandstorm (slightly forked) for Android as well. And then Radicale is an CalDAV/CardDAV sync server, which will work with a lot of contacts and calendar apps.

Given the variety of applications one can host on Sandstorm, it's hard to have an all encompassing 'Sandstorm app'. And for what the core of Sandstorm does, the web interface does it well on mobile.