"Serving cunt" is queer ballroom slang meaning to look extraordinarily stylish and feminine. Despite the strong word, in this context it is a high compliment, not an insult. If your teen uses it they are using queer drag vocabulary, often about pop stars. Decide based on your family's comfort with the word. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.