To me it looks like at least half of the perceived difference is just an optical illusion because of the big black connectors on the bottom side.
This optical illusion is enhanced more by light connector on the top and more dark areas on the bottom.
The difference (in the green) between the center and edges of the PCB is also already more pronounced than the difference between top & bottom.
The goal of KiCad's 3D rendering is also to give a visual impression and spot errors on the PCB easily, for example SMD parts on the wrong side, or mirror image text. Such things are easy to spot in the 3D view, but can be hard on the 2D top view of the PCB.
For any practical purpose the 3D renders look plenty good to me.
If you want photo quality renders, it migt be a better choice to use step-up to export the 3D representation and use external software that is more specialised for this kind of thing.
Adding solder fillets would also make a much more appealing 3D view, and they may even be usefull if they are volumetric correct from the stencil thickness & holes. And of course you also want to see how much solder is sucked up by those via through pad holes, and the solder dripping off on the other side...
As far as I know KiCad has no support for tented / plugged vias yet, and those features would also be usefull to inspect on a 3D render, but do not need photo quality renders.
KiCad has been improving quite rapidly in the last few years, and the rate only seems to be accelerating. Maybe the desiners have some time for thinks like this in 4 or 5 years but for the time being there is still plenty to do in polishing the overall gui and adding sometimes even basic functionality.
http://docs.kicad-pcb.org/doxygen/v6_road_map.html