Hi EuroGuy,
You’re correct that as a general rule natural fibers will tend to be more moisture and temperature regulating than synthetics and will provide a better microclimate for sleeping. Polyester also has more issues with static as you also mentioned.
This will depend to some degree on whether you are buying a topper, a mattress pad, or a mattress protector (the difference is that each one is thicker than the next). All the layers in the upper part of a mattress will have some effect on ventilation and temperature and moisture regulation (see post #2 here) not just the top layer but the layers closest to the body will have a bigger effect.
Phoenix