Should CAN bus in a vehicle be isolated?
The issue is not whether your system is critical, but whether there are any critical functions that will fail if the bus is disabled (made unavailable for any communication) by a fault on your "non-critical" device.

A bus means that all devices share a common channel of communication. If your device holds the bus in a non-idle condition, it will prevent all other devices from using it (or more precisely, that segment of it).

In general, vehicle systems are isolated to both: (1) isolate failing bus devices from impacting the remaining bus-attached devices and (2) prevent high-voltage transients and other power/signal issues that occur in vehicles from damaging bus-attached devices.