There's several key metrics that are probably good to monitor. These are already summarized on the Application dashboard for all mobile apps. Such as: User Experience, Crash rate, Network Performance, User Actions and Users.
This is all easily accessed from Applications -> <Mobile App>.
We have been keeping an eye on everything. The mobile app being the most important, since most of the users will utilize the app rather then the desktop. Ensuring that application servers have enough CPU and memory to handle the growing work load, as well as keeping an eye on errors/failures and crashes.
One of the most interesting cases with my clients has been in internal applications, and understanding which factors most contribute to UX, when moving from the office to home.
For this we have analyzed how Apdex has moved along with network latency. There seems to be quite a correlation in this, and while not perfect, it is the major factor driving Apdex a little bit lower. Client was amused that we could say exactly what the impact had been in this case 😉