Yeah, it's too small for an ACNL QR. Probably just leads to a website or something social as mentioned before. Too lazy to check.
Not to mention the three blocks used for placement are placed in the top left, top right and bottom left.
This isn't a QR for AC:NL. The pattern is wrong. (I also tried to test it and it didn't work.) The picture isn't clear enough to see what site it goes to with my phone. So no, in this case someone did not get an Animal Crossing QR code onto the back of their head. Sorry.
I've seen this before though with the outcome, it was posted on tumblr months ago. It's like a painting (that's why there is only 1 qr code) of a baby but of course it is likely that it was faked.