Are TVS diodes necessary if device is not grounded? (i.e. portable)
If I'm working on a battery powered device that has no connection to
ground, is it still necessary to guard against voltage spikes?

Any spike or surge does not respect a non-galvanically earthed system. That system (be it battery powered or electrically isolated) will have capacitance to ground and a surge current can flow via this route.
If the circuit is small and has an earth plane and the inputs are only slightly robust you might get away with it but, the devil is in the detail of the layout and where the device is placed or how it is held.