As a standalone film TWOK is pretty strong. As part of something that even then had considerable history behind it, not so much. But that is my devoted-fan-from-childhood perspective, and I have a hard time faulting the movie on that basis alone.
Regarding the standalone thing, I'm not sure why anyone would have expected, especially back in 1979, for Star Trek II
to pick up on plot threads from the previous film. The original series had consisted almost entirely of standalone episodes that rarely if ever referred back to previous episodes. And most movies and TV series weren't all that serialized back then. Look at the Bond films, the Pink Panther films, etc.
And it's not like TMP ended on some sort of cliffhanger . . .