Found a workaround, was able to set the horizontal scrollbar to auto on feature attribute control.
ESRI.ArcGIS.Mobile.Client.Controls.ViewFeatureAttributesControl.ControlCreatingFeatureAttributes += new EventHandler<ViewFeatureAttributesControlEventArgs>(ViewFeatureAttributesControl_ControlCreatingFeatureAttributes);
void ViewFeatureAttributesControl_ControlCreatingFeatureAttributes(object sender, ViewFeatureAttributesControlEventArgs e)
{
((System.Windows.Controls.ScrollViewer)(((System.Windows.Controls.ContentControl)(e.Control)).Content)).HorizontalScrollBarVisibility = ScrollBarVisibility.Auto;
}