It took me a little while to make sense of this one. At the size the strip is rendered by default in my browser, it was hard to tell Shermy apart from Charlie Brown. CB is the one driving the sled, not Shermy.
Beyond that, I think the word "mush" is interpreted by Shermy as short for "mushy."
Snoopy sure looks happy to be pulling Charlie Brown's sled. His question mark in the last panel adds a slight extra punch to the joke.
