"Nova" is clearly "really something" (or possibly "crazy"). He could have said "She was really zoogbah, that one". Which, I think, is the only time that TOS used made up slang, wasn't it? (NO I'm not counting the Space Hippies.)
Mitchell's lines in the final draft script are "A walking refrigerator," and "She was nova, that one." Obviously, they filmed the first differently.
ASIDE: In one marked up draft of the WNMHGB script, after making himself "dead" for 22 seconds, Mitchell says, "I won't add that to my repertoire. For a second there, I almost lost control." A few lines later, when he becomes aware Dehner is holding his hand, he says, "Hang on, baby. For a while. I'm scared." And a bit after that asks, "What's happening to me?"
I think someone like @Maurice should educate us in what ways did TOS NOT sound like, say, The Dick van Dyke Show". Heck, how slangy was The Munsters or The Addams Family when it wasn't intentionally sending up then-modern slang?
I dunno that I'm qualified to educate anyone on that. I'm not a speech expert and can only offer observations about media portrayals, but I can make some comments that you may or may not agree with.
From the linguistics podcasts I listened to and books I've read, I think it's fair to say—in general—in polite company, people spoke more formally in up through the early to mid 60s.
Star Trek's writers, by and large, weren't part of youth culture, and they certainly weren't writing in the vernacular of that. Even
Laugh-In occasionally sounded like middle-aged people writing what they
thought the kids sounded like. I don't have data to back this up, but, to my ear, it was at the end of the 60s and in the 70s where US network TV really loosened up and tried to go for more slangy speech, especially in comedy programs, possibly as networks tried to attract the 18-to-34 and 35-to-49 age groups that most appealed to sponsors because they spend more on consumer products than other groups (a factor in the CBS "rural purge").
Roddenberry didn't like the saluting and other aspects of the military (he didn't like piping in and "captain on the bridge," stuff, for instance), so that formality was out, but as he also saw the starship crew characters as professionals akin to astronauts, he was possibly reacting to the public perception of how they were seen by the media in his depiction of their manner of speaking.