A NMOS cannot charge the load completely to VDD and PMOS cannot discharge the load the VSS.
i.e.., NMOS charge the load to VDD -Vtn and
PMOS will discharge upto Vtp to the ground
True, this is a bad idea of buffering a signal, but it is wrong to say that pmos will do nothing, It ofcourse will discharge the load upto vth. And it is again a bad idea of making this circuit an inverter.