Well, you have to look at the historical context when viewing old shows like that to understand better:
“Counter Earth” was an ongoing theme in Sci-Fi of the 60s, just like “Multiple Universes” is an ongoing theme in Sci-Fi today (for the last 20 yrs or so).
One day they’ll mock the multiple universes from our era, like we mock the counter earth of that era. Such is how it is.
In fact, if you want a fun brain challenge Tristin Lollar – here’s an activity I’ve been doing since I was a teenager: I look for things that are faddishly popular in science or science fiction (because I like the genres generally) and try to figure WHICH one of these things will they laugh about in the future.
What they WON’T laugh about is probably pretty good stuff.
The rest of it? Mockable today.