>>49616663
The bare head is like Renko's naked body. The most overt, vulgar form of lewdness she can have. That is to say that her hat are normal clothes, meaning they are there to give a more chaste version of her beauty; whilst the ribbon is a small ghost of decency, there to entice men into wondering what it is trying to hide, much like lingerie.
In conclusion, the bare head is a vulgar display, whilst the ribbon is an act of pure, teasing eroticism.