Do mobo's also need to support dx 12?

GPU only. Graphics Processing Unit.

When you see game requirements they generally only focus on FOUR things:

1) GPU
2) CPU
3) System Memory (do you have enough... the DDR3 sticks on the motherboard for example)
4) Windows version (usually W7 32-bit or better but some games require 64-bit OS now)

*The motherboard is basically the platform to connect all the other pieces together... the memory stores the game data, the CPU processes that, some commands are sent to the GPU to be processed and finally the GPU video goes to the monitor.