Personally, I believe using MERN as a static site generator is in fact an overkill.
The reason being the requirements to achieve a static site generator — at the core, and by definition, all a static site generator does is to churn markup templates, to produce static HTML files.
Now, you can achieve this with MERN; but you will be left with unnecessary bloat — mongoose; express; code written, and optimised for server side rendering; are just a few things straight out of my head.
As a trivial example, what you can do though, is to combine Gatsby with MERN, and build a service for people who want to effortlessly host their static sites.
Hope this helps! :)