Getting LOR from mentor at an open source organisation
Full disclosure: I am not a faculty member.

To answer your first question - does it matter if you've never met your mentor in person? - the answer is: probably not, so long as you've worked together long enough or closely enough where they can still write a strong letter. It's unusual, perhaps, but so long as the letter itself is strong, I can't imagine there being an issue.

As for the second question - how much importance does the admission committee give to recommenders from open-source organization? - the answer is likely: no more than they'd give to any other recommender. Granted, if you're applying to a computer science program and have a recommendation from Richard Stallman, it probably looks better than a random developer on the internet.

But that gets to a much more important consideration: what qualifications does your mentor have? Admissions committees (to my knowledge) give more weight to recommendations from highly-qualified sources, such as another faculty member or an experienced professional. If your mentor is just a random developer on the internet working on an unknown project, there letter may not be considered as important as a letter from a faculty member who, while perhaps relatively unknown, at least has some publications.