This is an example of the China Poblana, or Chinese Pueblan dress style considered the traditional style of Mexico now. The model is standing in front of the University of Mexico building, intricately detailed in its own right.
The description is quite simple:
Es la Ciudad Universitaria de Mexico (Typical China Poblana costume)
The postcard was mailed, and the airmail stamp used is from a series printed in the 1950s.