What does "punch line" mean?

It's a figure of speech called synecdoche : part of something is used to refer to the whole thing.

In the above context, a punch line is a joke as a whole, not being the final part of it.

p.s

It's a bad usage of synecdoche.

Darn puppy peed on the floor again, got anything to clean it up?

Sure, use these pets.com shares.

Not a very funny joke, but it is one where a dot-com era's IPO is literally a punchline.

If you publicly fail at something, you may become the subject of some cruel jokes. In some of them, you or your project might be the punchline of the joke itself.

Of course, if you publicly succeed at something, you may be the subject of some cruel jokes too, but you're likely going to be better known for that success.

To say that something or someone has become a punchline, suggests that their only remaining relevance to anybody is as something you can end a joke with. The humour value of your failure being the only reason people have left to talk about you, you have become a punchline.

It was probably quite a striking idea the first time someone said that someone had become a punchline, but it's a tired and overused cliché now, even when used in better sentences than that in the question.

Aside from the definition of ‘punch line’ in Oxford Advanced Learners Dictionary - the last few word of a joke that make it funny, we used the word, ‘punch line’ very often in the sense of the clincher, most selling phrase in the ad or commercial message.

For nearly 40 years I spent in Tokyo operation of New York-based ad agency as a copy writer and later account executive, I kept being driven by severe clients (mostly American such as Procter Gamble and American Express) who had asked almost every day “What’s the punch line of the message you propose? Does it sell?, Are you sure?” We overtaxed our brains out to come up with a decisive punch line of the ad for the clients. I'm haunted by that word, 'Punch line' even today like a nightmare.

A punchline is the line that knocks you flat. Hearing the punchline is when the joke hits you. Without a good punchline at the end, the joke won't work. It won't have any impact.

Similarly with a business proposal. It helps to end with an impactful punchline to make it memorable to the person that it's pitched at. Hopefully it's a knockout and they're floored by your amazing expertise and prowess! They sign you up to pitch for their team :)

"One of the few dot-com initial public offerings that didn't end up a punch line" simply means it's one of the minority of such floatations that didn't end up as a bad joke.

Most IPO's failed so spectacularly that they ended up in every comedian's act, as examples of the gullibility and folly of those who invested in them: they had attained such a bad reputation that they were literally the punch-line of the jokes.

A large majority of those IPO's were laughably bad investments, in businesses which had never come near to making a profit, hence as offerings they were fated to also be viewed as bad jokes by more prudent investors, who had given them a wide berth.