Please give us some context. Where did you hear or read this, what was the general topic of discussion and who was talking.
Thank you for the context shema
It's hard for me to say what a 17-year-old male New Zealander might have meant! Does "Don't be a party girl'" mean stop being one or please don't become one. A form of kidding, yes we hope so.
'A party girl 'has several levels of meaning. First, it means a girl who has a very busy social life or more often, one whose only interest in life is parties or having fun, somebody who has no serious interests and no 'real' job.
It can often mean or imply a girl who drinks too much, uses drugs and sleeps around.
I admit that I am not very good at reading T-shirts! Even when I know what they mean I often wonder what the point is. A younger person could advise you better about wearing it.
Thank you for your comments.
In Singapore and Malaysia, a "Party Girl" is a local girl who dates a lot of foreign men, usually that she meets in bars. It doesn't really have anything directly to do with "party", it's more of a derogatory term referring to somebody who is promiscuous... So yes, hopefully it was a joke! (Unless it means something entirely different in NZ )