Not sure. I guess it's useful as a reference implementation (if it conforms to the specification that is), but not beyond that. Actual editors and office suites which could potentially use the code aren't written in C#.

There's a pull request for a basic mono build file - https://github.com/OfficeDev/Open-XML-SDK/pull/3

The repo doesn't contain any P/Invoke or C++/CLI, so I don't see any reason why it wouldn't work with Mono.