The picture shows a beautiful girl that happens to have DS.
Potato is a term that is used to infer confusion, low quality, unintelligent, unapealing.
So the picture pokes fun at the fact that even though the girl has Downs, she is still hot and most would bang.
Huh...
I was figuring it was some sort of "Hot Tottie" jail bait joke since the girl looks like she's 12.
I'm assuming she's some how internet famous and is actually in her 20's but the downs makes her look 12.
Not really a funny one. Not because it's mean or tasteless, but because it needs a half hour explanation.