Mister Unnatural says:

Keep Off Truckin'

What are they actually trying to forbid, if I may ask?

To enter somewhere, “because it’s dangerous“.

I wonder if the sign author was counsciously referencing the pop icon for humor, or if they simply used some clip-art.

Yeah, it's basically a more cheerful version of DANGER! KEEP OUT!

I wondered the same thing myself, about the source. But they can't just show a figure walking and strike it through, because you're allowed to walk past: they need that extra dimension, and I suppose there aren't that many ways to do that with a 2d character.

『ピクトさんの本』（内海慶一）を読んでみませんか？　面白いと思います。