A yard mullet is what happens when you mow the front yard one day, but leave the back yard for another day. This leaves the grass in the back yard longer than in the front, giving your yard a mullet. It gives your neighbors the impression that you are a master of lawn care, when you are actually sporting some serious "party in the rear" action. Acceptable reasons for a having a yard mullet include lack of time, lack of motivation, the need to drink beer with one's friends, or any combination of these.
I mowed the front yard yesterday, but (it got dark, it got too hot, I got drunk) before I could cut the back. I'm rocking a serious yard mullet right now.