Is it modest to talk about modesty all the time? Or, better said, is it helpful to discuss skirt lengths over and over, if the goal is to get people to focus on the character instead of the body? I think not. This is why (feel free to add to the list).
First, if modesty is misrepresented as being only about skirt length and measurements, the idea behind it is lost. I can dress totally immodest within the borders of tzniut measurements, it is near impossible to define "too tight" for example. On the other hand people can be totally modest while not complying with every measurement.
Second, if you talk about a topic all the time, you make it an issue where otherwise there might not be one. If I order you not to think about a kangaroo, you will certainly think about one. For some protest here, people chose the color light green as their identifying color. Before this protest, if you would have asked me if my colleague wore light green yesterday, I wouldn’t have been able to answer you. But now I can, because as soon as I see the color, the protest is on my mind. The same happens when you talk about skirt lengths or colors all the time.
Third, it can damage the self-worth especially of teenage girls. If you are told to hide your body, you could come to see your own body as something shameful (you don’t have to, but some girls might). Of course I am not saying dressing in tank-tops does not damage self-worth. It is in general the connection of self-worth with a way of clothing and being judgemental about it. We need to teach girls to accept their body. Yes, also, that their body is theirs alone and not to be displayed to everyone. But not out of shame, out of respect for it.
Fourth, it objectifies women. If the only time we talk about women is when the topic is modesty, something is very wrong. If a woman is reduced to what parts of her body are covered, it just as objectifying as if she is reduced to what parts of her body she is showing.
A final remark: I do not think the answer is to drop tzniut completely. I dress modestly and I would wish that my hypothetical future children (of both genders) did as well. But I think we are going about this the wrong way.