Other memes might present some fictional dialogue as setup and context before the image or final line or quote or whatever at the bottom delivers a punchline.
"Nobody: " could be read as “Nobody said nothing”. Sometimes further exaggerated with extra lines of hyperbole about how absolutely nobody said a single goddamn thing.
Which, in theory, serves to emphasise that the punchline comes out of left field. An absurd non sequitur with no setup, no context, and no-one asking for it. It’s not a response to anything, it’s just an inexplicable bit of nonsense that came out of nowhere.
But also it’s really easy to tack that on to just about anything that would otherwise have just been a mildy funny image, in a lame attempt to heighten the comedy of it. So it gets overused.