@Minus  
It is meta.  
Maud begins the comic by announcing that she is going to tell a joke (the comic’s beginning implies that the creator is about to tell a joke).  
Spike asks if the joke will have a punchline (at this point in the comic, you would wonder where it is going).  
Maud replies, “no.” This follows the rule of three (that a joke should have three “stages”), it is also abrupt compared to the earlier statements (once more, the rule of three, because the third frame breaks the pattern). This frame is also a statement to the audience announcing that there will be no punchline.  
The fourth frame is blackness. Once more, the rule of three is followed (conventional, American comics have only three frames. Japanese 4koma use four panels, but the fourth panel is supposed to be a post-action panel, reacting to the “event” which occurred in the third panel). Maud already announced that there will be no punchline, and so there isn’t one. As @nitro27  
said, there is a punchline, the punchline is the absence of a punchline. The definition of irony is the reversal of expectations, but you, as the post-modern subject, have been trained to expect irony. You expect the unexpected, a sudden change, but instead the comic proceeds in a completely straight line, leaving you baffled by something that makes no effort to confuse. The surface value is all there is.
 
The characters lend themselves to this situation, since Maud is Pinkie’s relative and breaks the fourth wall in this comic, but she also completely lacks any sense of subtlety or even the implication that she sees past the surface level of things. Spike is often used as a stand-in for Bronies because he is the only male character in the show. Or maybe the characters are just happenstance and the whole thing is a stupid fluke. Maybe the surface level is all there is, or maybe it isn’t.

I don’t get it…

@nitro27  
Same!

First of all, the hell kind of question was that? Second of all, that joke did have a punchline. Third of all, I lol’d anyway.

You know what joke did have a punchline? Jonestown.