MirageOS, at least from my viewpoint, is trying to figure out what the cloud software stacks of tomorrow should look like. Rump kernels give you production quality building blocks for running existing code today in any environment. I wouldn't compare the two projects too much.

The two projects are very complementary. You could easily write a MirageOS rump kernel backend that linked to the kernel device drivers, and build higher level services (HTTP, TLS, XMPP) in OCaml that use the production-quality NetBSD device drivers.

It's all good fun in library OS land :)