We haven't decided on that yet, we might decide on it over how high her "Horny level" or some other way, we are open for suggestions on that end!
I think giving the player the choice on how their Caly would feel about it could definitely be more engaging. The choice could be something introduced in the intro, "How does Caly feel about strutting around in the buff? 1. Horny 2. Neutral 3. Shame." Then with this could come some side effects. If she's horny every slap, grope, or acknowledgement would add to the horny meter and her expressions and perhaps some interactions would be played out accordingly. If she's neutral about it then slaps, gropes, and acknowledgement could perhaps just make her smile knowing that others are definitely enjoying her body. And if she's ashamed then her movements and expressions would be more as if she's trying to contain or cover herself. Slaps, gropes, and acknowledgement could also go up but not as much as if she found it horny (shame kink).
Could also have this affect gameplay as well. If horny meter goes up if she finds it arousing, then perhaps she loses the battle more easily. If she's neutral and her horny meter is up, instead of getting aroused by walking naked she instead gets a small attack boost. If she's ashamed it could be a bit different, if her horny meter isn't up she gets a slight boost that allows opponent attacks to miss, but if the horny meter is up, then nothing is different from the regular gameplay.
Certainly a more involved and complicated system, but if you'd like it to affect gameplay then this might be an option.
If you'd like to keep it simple however, I still think allowing the player to choose how their Caly would feel about it is a good call. Allow the player choice and the choice will determine how the horny meter is affected. Horny, the meter goes up fairly quickly. Neutral, it doesn't go up. Shame, it goes up slowly. And if you're willing to take it a bit farther, add expressions accordingly based off the player choice.