When you watch it today it has pretty strong 'eighties trash' vibes.
What does this even mean?
80s trash ala what passed for SF in the early 80s like Knight Rider, Airwolf, etc... probably.
Both Trek II and III have a decidedly TV-movie feel to them. Khan did try to be a bit more epic and cinematic, but at times the lighting in both has that flat TV-series sort of vibe going.
The ancillary actors mostly came from TV and either were already stars there, or went on to be best known there later. Montalban was best known for being Mr. Roarke on Fantasy Island even though he had already established a long film career. Judson Scott and Kirstie Alley were really TV actors. Then with Trek III you had Christopher Lloyd who at that time was best known for Taxi, playing the heavy, with a heavily-made-up John Larroquette of all people as his Klingon sidekick.
Despite the wide-screen aspect ratio, they definitely feel at-home on the TV screen due to these familiar TV faces. The one thing that kicks them up a notch is the use of ILM. The loss of ILM is a big reason why Trek V looks like crap. The use of ILM again in Trek VI helped mask the fact they were, by that point, reusing and redressing so many tattered assets.
It wasn't until Trek VI that movie-actors entered the picture, via Christopher Plummer and David Warner.
The entire Trek film franchise is one of attempting to be a bargain-basement blockbuster. Outside of TMP, the gamble that started it all, the success of these films was never assumed. The studios didn't greenlight what I would consider to be a blockbuster budget for any of them after TMP until Nemesis, which backfired, and then the 2009 film, which paid off.