Some gurus of long-form improv will tell you not to make jokes, and the reason is because jokes get a laugh, but kill the momentum of the scene.
Good point gurus, but sometimes a joke is so good you just have to make it. And, some games like freeze or time-jump are really conducive to jokes. But, if it's obvious that somebody has just made a joke on stage, please don't let them keep trying to form a scene.
I've seen the following way too many times in the game FREEZE. Don't do this: Somebody freezes in with a funny idea. The awkward position is the set-up, and the first line spoken is the punchline. Then, nobody tags the person out and so two people are left trying to establish a scene based off a joke, which is hard to do. The laughs progressively diminish, and the scene flounders. Then, people become more reluctant to freeze in because they're worried that's going to happen to them, and people take less risks. The following freezed-in scenes get more and more boring.
So, what's the solution?
It's ridiculously simple. If you see somebody make a brilliant joke that gets a big laugh, and it's clear that it's a gag and not a game (a gag is something that really only works once e.g. coming into a scene in freeze and making an unexpected comment; a game is something that can be played with and repeated e.g. one-upping each other), do this--EDIT THE SCENE.
I guess it's sort of like asking somebody out on a date. It takes a little set-up, but once the question's out there and the answer's been said, you don't hang around too long because it gets awkward really fast.
But what if I can't think of anything to do?
a) too bad, this is improv not writing a book or commanding troupes
b) it doesn't matter, somebody just got a big laugh, what the scene needs now is to quickly transition and start building again.
Here's a formula that works for games like Freeze where jokes and sight gags are part of the game:
1. Set up a scene, a good scene with actions and objects.
2. Sight gag occurs.
3. Somebody freezes in as soon as those gags get a laugh, and start #1.
This is tough for improvisers because a lot of us only want to start a scene if we know we're going to do something funny. But, if somebody else has done something funny, it's time to let that glow, and let it end well.