Why is the title Punchline? How does it relate to the series itself?
According to dictionary.com, punchline means
the climactic phrase or sentence in a joke, speech, advertisement, or humorous story that produces the desired effect.
So do we have to wait until the end of the series to understand the meaning of the title?
The title Punchline in Japanese パンチライン contains the terminology [パンチラ]{Panchira}, which means "the instant of miracle granted by the god of chance when the panty becomes partially visible" according to Nico Nico Pedia, or simply panty-shot according to Wikipedia.
This is inline with the theme of the show, where the main protagonist has to avoid all those moments when the girls inadvertently expose their panties, or he will become excited and somehow cause an asteroid to collide with the Earth.
The relation between the title and the term panchira is prominently displayed in the OP of the series, and also in the lyrics of the OP 「しょこたん♥でんぱ組」:
パンパンチラ パンパンチラ パンパンチラ パンパンチライン
パンパンチラ パンパンチラ パンパンチラ パンパンチライン[...]
Why not look at my panty line?
女の子に恥かかせないでね
The line before last in the lyrics also gives rise to another interpretation of the title: Punchline = Panty Line, since the sound of panty and punch in Japanese is very close （[パンテーィ]{Panti–} vs. [パンチ]{Panchi}）.