I heard it was because of the Hey Jude scene, but since CBS released the first season in California for Laverne and Shirley, and they eradicated the usage of "Yesterday" (Beatles problem) AND eliminated an important scene because of other music references (getting rid of an entire scene/joke "problem"), I guess they could try.
I doubt the one song is holding up the release. That was a rumor started by someone who has no inside knowledge of anything CBS. Since songs/scenes were removed from other shows it seems they would probably opt for that with Mork & Mindy. With Robin Williams returning to a CBS tv show this year I'd say now seems like a really good time for the final Mork & Mindy release. Hopefully we'll hear some news soon.