Disabling a device on Hubitat only stops the driver code from running (which means neither manual nor automated/app commands will work and commands/events from the device get ignored). If guess there's minimal overhead, almost certainly less than the alternative of executing the driver code, though the implementation details aren't public.
If this is a mains powered Z-Wave device, it's probably a repeater, and your mesh may not be happy without it. Best practice is to correctly exclude it. Battery powered devices are unlikely to be a concern, but in any case, the disable feature just relates to the Hubitat device/driver, nothing special to do with the underlying protocol or mesh network.
