To let your hair down means to relax and enjoy yourself, but in this context the speaker wants the woman to worry about being dignified
Context: Because you're a smart, pretty woman who I suspect is a lot of fun once you start worrying about being dignified or behaving correctly without just relaxing or indulging in too much enjoyment.
Hypothesis: Because you're a smart, pretty woman who I suspect is a lot of fun once you let your hair down.

Is the hypothesis incorrect? (Yes, no, or unknown)?