"Man" is "one", as in "One has to follow the procedures." But in English it is often now replaced with "you": "You have to follow the procedures, first you do this, then you do that..." even if you are not talking to a specific person about them specifically doing all that.
That sounds odd to me -- you would typically put something at the beginning of the sentence if it's the topic: something that you want to talk about.
But "no jokes" is not really anything you can talk about, because they are something that doesn't exist ("no").
So having them as topic seems wrong.
In English, in the right context, you could also say either "people don't make jokes about that" or "you don't make jokes about that, or even "we don't make jokes about that" to convey the impersonal instead of "One". Incidentally, I lost a heart for putting "jokes" in the plural. . .I suppose Duo has to know whether I know that "keine Witze" is actually singular, but it's a bit harsh sometimes . . .
I feel like this phrase is odd enough that it deserves the mechanic that shows you the whole phrase when you click on one word. My 1st choice definition for each of these words did not make a sentence. My husband guessed when i said each of the words individually... But I certainly didn't.