You are right that the Lord didn't have to tell the women how to dress. He was preaching mainly to Jews who had instructions from the law about what nakedness is.But is this the best way to approach the problem? Christ in the Sermon on the Mount (Matthew 5:28-29) placed the responsibility on the men to accept the blame and deal with their problem. There is no record of Christ telling the ladies to "cover up." For Him to propose a 90% rule for body coverage probably would not have done much good, in a society where probably almost all women were already covered up in long, loose-fitting robes, but the men were lusting after them anyway.
The Bible speaks much about nakedness in the OT.. Much of it is about the nakedness of men, but I would imagine if something was naked on a man, so it would be naked on a woman as well.