Emo Grass

Alright listen up, you absolute walnut. The joke is that "emo" is a subculture associated with emotional and often self-harming behavior. The person wishes their grass was "emo" so it would cut itself like an emo person might do. It's funny because grass can't be emo, and it's a dark joke about self-harm. As a listener, I bet you still cut yourself with dull scissors.