I don’t want my staff to take their attendance using the Truein User App on their mobile. Can they use the App just to view their attendance?

Yes, the User App can be set to enable just viewing of attendance. 

In Truein Web Dashboard:

Web Dashboard > Staff Directory >Select Staff > Action Column > Click Three Dots > Click Edit > Access Tab > Unselect  ‘Allow Attendance from User App’

Select Invite, assign the required role and Save.