I think it is important... actually I think that the Python libanki should have much more documentation in the first place, but I guess that won't change soon. Meanwhile, the Java libanki should do better than that.
Most Java developers are used to proper documentation (especially in such core classes) and very critical of classes that contain zero documentation.

I would even go further and say that each member should have a Javadoc comment. That would be extremely useful for developers who don't remember exactly what are `mMod` and `mODid` for instance. It is hard to believe there is zero documentation for these cryptic fields. If you think it makes libanki merging more difficult, class-level javadoc might be enough though.