Because you didn't provide surrounding contexts, I googled the sentence.
It (first, I assume) appeared in the cartoon "The Simpsons."
According the lines, I gather that it is a metaphor, which might be pejorative.
Hi! buttercup=?Eh, you silk-wearing buttercup!
In the sense used by Groundskeeper Willie, he was comparing Principal Skinner to a flower, and insinuating that the principal was effeminate.
Outofdejavu & Ouisch, Thank you for your help.
Ouisch, If you have some idea, could you please explain why the gardener -Willian call the principal 'buttercup' rather than other flowers such as rose, lily, etc. when they didn't stand in front of butterup flower at that time?
If I said John Doe was a buttercup without any other context, could native speakers understand what I meant?