The current release is 0.21 which suggests that it is not ready for primetime.
It suggests it, yes, and to me as well. On the other hand, there is a common prejudice that something requires many releases, or frequent releases, to be qualified as good. While it might be true on average, this is not necessarily true: some packages need not so many steps for becoming very usable, mature or stable. (Note that I'm not asserting anything about Monotone, here.) We should merely keep an open mind.