Yep! Would be pretty easy since Ractive is supported via consolidate:
https://github.com/tj/consolidate.js.
Usually, though, when including a newly supported templater, I also update the "mimosa new" command to incorporate that technology as an option. I'm happy handling that part.
With Mimosa 3.0, about 2-4 weeks away at current pace, mimosa-server will be pulled out of Mimosa as a built-in dependency, which frees me to include more templating options. Right now, if you add ractive to mimosa-server, you are also essentially adding ractive to mimosa, and I don't want to build up the dependencies at this point. Ractive needs to become a dependency of mimosa-server in order to support Mimosa's embedded server.
So, the gist is, if you want to add it to mimosa-server, you can, and once 3.0 releases, I can add Ractive to the "mimosa new" scaffolding. The amount of work to add it is fairly trivial. Needs to be in the package.json, needs to be validated as ok. Then verify the internal and external versions work ok.