What causes this surface texture, and how does it affect flight characteristics?
I believe this is buckling of the fuselage, due to heavy compression loading.
Buckling is the process of out-of-stress-plane movement which happens under compression. The below image shows it well.



Source

More info on buckling here.

Here's another picture, possibly of the same (type of) aircraft, with the description saying:


  It appears that the fuselage contains a very heavy cargo, the weight of
  which is reacted by significant overall shear loading at the axial
  station of the landing gear. The overall shear loading is absorbed by
  the skin of the fuselage, which has developed significant
  post-local-buckling diagonal tension between internal ribs and
  stringers.


This seems plausible to me, it could be that upon a rough landing, there was excessive compressional loading exerted by the landing gear, which caused the buckling. 

I'm not an expert on airworthiness, but usually buckling indicates that a part is loaded more than it's supposed to be.



Source

Other aircraft have it too, see  this question with a further explanation about buckling.