When running in the Mendix cloud it is not possible to access the database directly:
If you would want to use the data available in your Mendix app you could create a REST api or such to access your data.
Xarmin.forms, as I understand it, is a way to build a mobile native UI by connecting some front end development tools into a database. In that way, Xarmin.forms replaces the page builder part of Mendix for building some reports or pages while still having the Mendix app do business logic and database.
Xarmin looks like it integrates with REST and SOAP just fine, so the way I would do this is to build your Mendix app, then create an exposed REST or SOAP Web Service that you can hit to get the data points you are interested in. It might just be easier to build the mobile page in Mendix, however, as it looks like you would have to maintain pages and a data model in both Xarmin as well as Mendix.