Yes, I think it is a sin against modesty. The Catechism of the Catholic Church says we are to dress modestly, both men and women. Now, a lot of us then get all hung up over the definition of the word “modesty”, unable to come to a consensus, throw up our hands, give up, walk away, saying we don’t know what modesty means! We try to say it’s too difficult of a word to pin down, define, conclude (wrongly) that modesty doesn’t matter!
(“They will ridicule Christian simplicity; they will call it folly and nonsense, but they will have the highest regard for advanced knowledge, and for the skill by which the axioms of law, the precepts of morality, the Holy Canons and religious dogmas are clouded by senseless questions and elaborate arguments”.Venerable Bartholomew Holzhauser" (17th Century)).
Now, the Bible also says, repeatedly, about the need for modesty in dress…Timothy, Corinthians, 1 Peter, etc. Job even had a sort of contract with his eyes.
Further, many of the saints have told us, again and again, about the virtue of modesty.
‘When she was thrown into the air by a savage bull in the amphitheatre at Carthage, her first thought and action when she fell to the ground was to rearrange her dress to cover her thigh, because she was more concerned for modesty than pain.’
Pope Pius XII, Allocution to the Girls of Catholic Action, speaking of St. Perpetua
Now, there are the virtues…chastity…goes hand in hand with the fruit of the Holy Spirit, modesty.
Well, there’s an accepted apparition of the Catholic Church, Fatima. Our Lady of Fatima does try to show the 3 children hell and what it is like. Then, she tells us sins of the flesh are the main reason people go to hell. Now, many people seem to see absolutely no connection between these sins and immodesty.
Woe to women wanting in modesty.
Our Lady of Fatima to Jacinta
Let me give you one quote by a saint on immodesty:
So, in a nutshell, we must not dress immodestly, look at those who dress immodestly, or encourage those who do if we want to do what is pleasing to God.