A cupcake someone has licked the icing off of and throw away.
It means no one likes you they just like what comes with you.
“Sarah is such a trash muffin, I only like her for her boat. “
“Alayna is a trash muffin but it’s better then being alone during lunch.”
“I think i’m just a trash muffin to Paige and Reagan. They only ever hang out with me when they need a ride.”