Angela Cartwright stated it was her worst one. Even though a teenager (she was 16-or-so when it was filmed), she absolutely loathed rock-and-roll music. She had never danced to it, and in the scenes where she had to do a "groovy" dance on a pool table, she didn't have a clue on how to move and somehow just had to come up with it.
She was being way too hard on herself because as I recall she did a good job with the dancing.
playing not only Dr. John Robinson, but his own evil self who crossed over from a parallel dimension
Having possibly seen that LIS ep as well as maybe the Star Trek ep
Mirror, Mirror is probably why many DS fans had no problem accepting a parallel 1970 and 1841 when DS did those storylines. I know it's why I instantly knew what they were doing...