A cupcake slang for a girl is "a young, attractive girl" or young and attractive woman.
In slang cupcake for a girl can also be used as a term of endearment that is similar to "honey" or even "sweetie".
Although it's important to be aware that some girls and woman may perceive the term cupcake when said to them as condescending or even objectifying as it can also focus on superficial qualities.
As a term of endearment the term cupcake can be a playful and even affectionate way of addressing a woman or girl and suggest that she is sweet and delightful that is similar to calling the girl or woman "sweetheart".
As slang "cupcake" can be used to describe a young and attractive woman, but the usage might be considered to be objectifying by some woman.
And yo have to be careful because the term cupcake can sometimes be misinterpreted as while it can be used innocently, the term cupcake can be perceived negatively, and most particularly in any situations in which there is a power imbalance or if the recipient feels it is condescending.