Yes, for the performance stuff, Java skills are important.
But you can come up with your own ideas for things to do as well. The ideas page is just to get you going. Ruboto has matured enough so that implementing libraries on top of it would be very useful. There are many Android APIs that could be made much easier to access using a Ruby DSL.
You could also browse through the Ruboto issue tracker and see if there are ideas you can pick from there:
https://github.com/ruboto/ruboto/issues
Bottom line is that you can invent any idea for working on Ruboto in JRuby GSoC 2013 :)
--
Uwe Kubosch
http://ruboto.org/