Is Vout in this circuit loop-grounded when Vin becomes 0V?
No - the output MOSFET will become open circuit when Vin is zero.

The output will be floating. The actual voltage at the output will depend upon the load. If there is something like a relay or lamp that conducts to ground the voltage will go to zero. As shown the capacitor C will charge up when Vin is a logic 1 and then retain its charge at 3.3v when Vin becomes zero.

This assumes that the threshold voltage of the MOSFET is adequately low to turn the device on with the ~3V available.