Knock Knock / Who’s there? / Alex!
Alex the questions round here!
This is actually not that bad - for a knock-knock joke.
I'm not saying that I like it, just that it is better than most knock-knock jokes.
There are two critical elements in this joke - the pun centered around the word Alex, and the surprise that the listener receives if the joke is delivered in a forceful manner.
The pun is between the name Alex, and the words "I'll ask" - as in "I'll ask the questions round here"
This joke is also a bit meta, or a little bit aware of the text of the joke itself. Most knock-knock jokes end in a bit of a nonsequitur, but this one at least knows that the prior statement was a question.