It looks like a poorly written text message. ^__^;;; The person is asking, "WHY (Y) are YOU (U) so stupid?" It's quite mean actually unless you know the person and they're clearly joking with you. In English, there are a few letters of the alphabet that sound exactly like complete words. In this case it is "Y" and "U". It's used a lot as shorthand writing in text messages. Hope that helped.

Oh yes: "Y U NO" = "why don't you...?" It's text-message format AND broken English.

There's also a running joke on the internet, called the "Y U NO" Guy. The writer was probably thinking of this as well. http://knowyourmeme.com/memes/y-u-no-guy