Given that the main use case is stuff like running machine learning tools, the only people directly targeting DX12 on WSL will probably be framework/library developers, who might want to add DX12 as one of the graphics systems they support. If that's the case, the vast majority of developers won't notice any difference other than more software supporting graphics acceleration when run in WSL (or, another way of looking at it, when Linux is running on the WSL "hardware"/"platform").

Sure, nothing stops you from targeting DX12 directly in your application code, but why would you do that? At that point, you'd just target Windows since your users would have to be running it anyway.