Marco was dressed as a princess and is Princess Marco in the interdimensional market. Princess Marco is a brand and Star is wearing that brand. She knows Marco so she is dressed like her friend. So whether or not Star is dressed as a girl or a boy depends on how Marco as Princess Marco identifies. Either way Star identifies as a girl as far as I know so it doesn't matter what she is wearing. Your mileage may vary. Do you know how many girls crossplay as Lady Phantomhive? Ciel Phantomhive identifies is a boy so a girl dressed as Lady Phantomhive is dressed like a boy.