The body is a temple of the Holy Spirit. How we dress shows how much we respect the dignity of our bodies as temples of God and made for eternal life, ourselves, others. It also shows what we think of the Mass. We are coming to the Sacrifice of Christ on the Cross made present on the altar. There is nothing informal about the Mass. The way we dress shows what we think not only of ourselves but of others. While we should not be motivated by vanity, whether male or female, we do well to “dress up” and to “dress well.”
One should avoid exposing too much skin. Please: shoulders and arms should be covered at least as far as somewhere between shoulder and elbow, i.e., dresses, shirts, blouses, T-shirts, etc. should have sleeves, whether short or long; legs should be covered to at least just above the knee; shoulders, chest and back should be covered; necklines should be modest (err on the higher side). Revealing clothing, whether because it fails to cover the body (e.g., midriff, cleavage, etc.) or is so tight that it is, in fact, revealing, (e.g., tight pants or T-shirts) is to be avoided. A light and comfortable scarf or shawl can be worn over the shoulders during Mass and removed afterwards when you might want to be less formal. Ripped jeans, sportswear, sweatpants, etc. are not respectable attire for Mass. It is possible to dress well yet comfortably.
Dressing immodestly is itself a sin. It is further sinful because it can lead others into sin. Ladies: you need to particularly understand the weakness of men in this regard. While men can delight in the beauty of a woman without any offense against chastity, immodest dress may become a cause of temptation.
Parents: it is your responsibility to teach your children in this regard and to be good models of showing respect for God and others. Please do not be surprised if Father steps in if you fail in your responsibility. Let us be respectful of ourselves, others, and the occasion: the Holy Mass.