I don’t understand the “Nobody” part, especially since in most memes it’s just blank. It makes sense when “Nobody” was saying something that most people disagree with, eg:
Nobody: I love slamming my fingers in a car door
Ford: New F-150 now comes with a dedicated finger-slamming door
That would make sense. It’s a joke about someone being out of touch with popular sentiment. But the ones where it’s:
Nobody:
Optimus prime taking a bath: Ahh, my electronics!
It seems like the nobody part doesn’t relate to the meme in any way, except for being a common format for presenting things.
When the “nobody:” part is blank, it means that nobody said anything, or nobody did anything.
As others have suggested, the meme means that some would say or do things without any trigger or nobody asking for it.