Yes it is. I don’t disagree with you.
I’m not 100% sure what mean by subjective. If you mean the viewer, you have to take into account the norms of that community. If you mean the wearer, although it is subjective, it isn’t helpful to throw up your hands and say this is subjective. It’s more helpful and empowering to say we should all try to sensitize ourselves in this area.
Loshon Horah is also subjective. The words I say, with the tone of voice and facial expression can make a statement a very positive one or a serious aveirah. But I am still obligated to watch my words and my facial expressions, etc.
So instead of trying to give rules here about length and style of skirt, when dealing with a diverse group of people, I am just trying to make the point that there’s halacha, and there is also a message I’m sending, so more people will use their head and pay attention to the message and improve in these areas.