Joe, think of it like "what is (this) compared to (that). "This" is "all the world does say." Not as in what the entire earth is saying, but "all the world" as in "everything the world says."
"That" is mirth, my friend, and pitcher.
In other words, what is everything the world says compared to mirth, my friend, and pitcher.
Clearer or muddier? Just think "everything" when you sing "all."