isn't the graphics card much more important than which mb you use for mining? [...]
Yes you are correct. The Motherboard is *generally* only a factor for how *many* cards you can run.
I disagree, the wrong motherboard can be a pain in terms of hashing stability speed, overall stability, setup time, etc.
Some motherboards fail to keep all cards connected, some need special drivers in windows, some need you to disable the onboard GPU and plug the monitor cable to one of the cards, etc.
Some motherboards like ASUS B250 mining expert has 18/19 GPU slots but you need to mix certain Nvidia and certain AMD cards in certain slots, annoying.