I don't know if this is my conservatism speaking or not, but one problem I see with this approach is that you wouldn't learn as much because more batteries are included. Having to implement more than just a kernel (like the standard library and userland) gives you more of a chance to learn intangibles like what is or isn't appropriate to include in an API or what programs will actually be useful given your overall design. Also, there could be the problem of not having as many resources available in the language you're writing your OS in. That's a huge deal for a less experienced programmer.

That all being said, Zig looks pretty okay. It would be a good candidate to write an OS if you had to because the standard library looks trivially portable so you could test the userland on a known working kernel before getting it working on your new kernel.