Hi GP,
There are many variables that work together that affect the sleeping temperature of a mattress (see the post I linked earlier) and each one has their own tradeoffs. Mattresses aren’t tested “as a whole” for airflow or temperature regulation so only looking at one of the many variables or materials can be misleading without looking at all the rest and how they work together. The most effective approach would be to look at all the materials in a mattress and avoid materials or designs that are known to sleep hot for some people (although they may be fine for others) in combination with the experience of a manufacturer or retailer that has your best interests at heart and your own “educated best judgement”.
Natural or “semi synthetic” viscose fibers or fabrics tend to breathe and wick moisture better than synthetic fibers but there are also some synthetic fibers or materials (such as coolmax or outlast) that do a good job with temperature regulation. Synthetic quilting foams or fibers are also more breathable than many other types of materials (such as memory foam). There is also more about different fabrics (including a comparison between cotton and bamboo or other viscose fabrics) in post #7 here and the posts it links to (it is discussing sheets but the same materials are used for mattress covers).
There is also more about wool quilting in post #6 here and there is also more about quilted covers as a whole in post #12 here and the posts it links to.
I would also keep in mind that the mattress protector, sheets, and other bedding that you use can also have a significant effect on the sleeping temperature of a mattress and in some cases can offset some or most of the temperature regulating benefits of the mattress itself.
Phoenix