Frequency response of a parallel RC filter (smoothing capacitor in rectifier circuit)
Does that look reasonable?


No because the diode only provides charging current during the part of the AC cycle that it conducts: -



So, the diode conducts during the charging period for a short part of the cycle and, for the rest of the cycle the capacitor is free-falling due to ONLY the load resistor,


  Or is there is better way of calculating the lowpass response of the
  smoothing capacitor?


Take into account what I've said above and develop an exponential decay formula that describes the falling capacitor voltage under the influence of the discharging resistor: -



Picture source