Yes, I like to read sources of big projects as a relaxing activity and almost never is documentation written in a way that a newbie can immediately catch up. The best projects will have a README.md in every directory that explains what a component or namespace is about, or nice big documentation headers. In my experience though if you really want to contribute the lack of documentation is only a small hurdle. If the code is well written then just following the code from the entrypoint on will give you a very good idea about what's going on. All the files and their names should make sense in under an hour or two.

In my opinion documentation is most important for external consumers of APIs who don't want to spend time grokking the code, and for explaining design decisions. For contributors it would be code quality.