What word goes between anticlimax and climax?

You could say the punchline was tepid, or one of its many synonymns:

(especially of a liquid) only slightly warm; lukewarm.

synonyms: lukewarm, warmish, slightly warm; at room temperature

"tepid water"

showing little enthusiasm.

"the applause was tepid"

synonyms: unenthusiastic, apathetic, muted, halfhearted, so-so, 'comme ci, comme ça', indifferent, subdued, cool, lukewarm, uninterested, unenthused

"a tepid response"

You might also describe the punchline as meh, depending on how informal your context is:

expressing a lack of interest or enthusiasm.

"Meh. I'm not impressed so far"

uninspiring; unexceptional.

"a lot of his movies are … meh"

Frame challenge: there is no middle ground.

Technically speaking, anticlimaxes are a type of climax (much like anti-symmetry is a type of symmetry). What makes them "anti" is the following resolution, not the climax itself. If this resolution trivializes the climax - which I would say is independent of whether or not it's disappointing, see any Monty Python skit - the climax is an anticlimax.

In other words, the word you're looking for is anticlimax. Particularly disappointing anti-climaxes (which I assume are what you aren't talking about) are what have the special names, usually implying that the author is a bad writer ("plot hole" is the first thing that comes to mind, but that is far too general).

To use your analogy, the climax of a joke isn't the punchline, it's the moment before the punchline, where the "victim" of the joke says "I don't know, why did the chicken cross the road?" or "Orange who?" or even just stares blankly at the joker. The punchline itself is the resolution, and thus defines whether the climax is "anti" or not.

That said, the answer to the question you ask in the body (which is different from the title question, for the reasons I have explained) is admirably answered by @KevinWorkman, and seems to be what you are looking for.