[nostrademons]

It's pretty mature, but its place within the Python ecosystem is different from JRuby's place in the Ruby ecosystem. Jython is basically an extension language for the JVM: it lets you use the familiar & comfortable Python language to script your JVM languages. Many popular Python packages won't work on Jython, either because they depend upon C extensions or because Jython is several versions behind CPython.

Ruby has much more of a bias toward writing libraries in pure Ruby, and so a greater portion of the Ruby ecosystem runs on JRuby. Also, Ruby's use-cases largely center around Rails and system administration, while Python also has large presences in scientific computing, scriptable C++ network servers, desktop GUI apps, etc, (none of which work on Jython). That makes JRuby very attractive for deploying a Rails frontend on top of JVM backend infrastructure, but the analogous situation in the Python world (Django webapp in front of say Twisted or Celery) won't even run on Jython.