I'm not sure that this will ever be a fair statistic because coffeescript should never really be used for shared / module code IMO. I love CS for my application code but would never release a package on github in CS, only JS.
Once you are releasing things outside of your controlled/team environment using anything but JS is just a loop for others to jump through.
I imagine a lot of projects have both CS source for development, with compiled JS for deployment. That might skew the stats between JS+CS, but it's a good indicator of CS' popularity compared to other languages.
Considering that, releases would be in JS, as they should be.
Maybe saying it's widely accepted isn't entirely appropriate, but it certainly is gaining a lot of traction.