The sticker does not need a long message.
The shape itself can communicate the idea.
For someone from Washington, seeing the outline can immediately bring back memories of evergreen forests, mountains, coastlines, ferries, rainy afternoons and outdoor adventures.
Turning the outline upside down adds a little personality.
Instead of displaying the most conventional version of the state, the owner is doing something slightly different.
It can essentially say:
“I know what this is, and I know it’s upside down.”
That is part of the joke.
The “Inside Joke” Factor
Perhaps the biggest reason the sticker catches people’s attention is that it can function as an inside joke.
A normal Washington sticker might not make anyone stop.
But an upside-down one can make someone ask questions.
“What state is that?”
“Is it supposed to be upside down?”
“Did they put it on incorrectly?”
“Why would someone do that?”
