Dressing modestly, to me, means simply dressing so that you are not an occasion of sin by your clothing or lack of same, for someone else (which is uncharitable and therefore sinful). Love/charity is throughout the Bible. Dressing is one way a person can be uncharitable/unloving to others, is all. Speaking is another. Actions are another. Thoughts are another.
Dressing appropriately has more to do with societal norms, including what a person has grown up with (as opposed to what they’d like it to be). It also has to do with Charity towards others and respect for the them - including, at Mass, respect, love and reverence for Him who you have before you.