nature's good in a cup
No, you're not right. Sorry.
First, I don't think it's using "good" as a noun but an adjective. I believe it's saying something like, "Nature is good (tasty) in a cup."
Second, even if it were using "good" as a noun, it could. "Good" is a noun as well as an adjective, e.g., "We tried to see the good in him." So "good" could be used to mean "goodness" as it does in this context.