It look like mingw is inconsistent with AArch64 standards, I think this should also be addressed with the upstream mingw project too.
First off let's have a look at what MSVC does; MSVC defaults to 4k pages for AArch64. So even if AArch64 in general can have 4k/16k/64k pages, it seems that Windows on AArch64 is not planned to run on anything else than 4k pages.
So for mingw, if it targets producing binaries that run on Windows on aarch64, there's strictly speaking no need for it to produce anything else than 4k pages. However, if targeting producing binaries that run on both actual Windows and Wine, then it could indeed make sense to use 16k or 64k pages. (And it has very little extra ill effects of course.)
I guess changing the default could be reasonable - if you have time, feel free to file an issue at https://github.com/llvm/llvm-project/issues for the lld component, and CC me on it.