Is RMS of a sound wave good measure of loudness?
No

The sensitivity of human auditory is very dependent on frequency and without any type of frequency weighting, a raw RMS estimate will be pretty much useless. 

There are some standard frequency weighting curves, most popular being the "A" weighting.  See https://en.wikipedia.org/wiki/A-weighting. The sensitivity is also dependent on the absolute level and hence there are multiple weighting curves. 

Sp frequency weighted RMS is a good starting points, but they typical ignore any sensitivity to time domain behavior like impulse, transient, masking, etc. There are good models to predict actual perceived loudness but they tend to be rather complicated.