First, it doesn't matter what particular thing a non-believer does. A non-believer is condemned because of non-belief, so whether a non-believing woman covers herself or uncovers herself is irrelevant. She is still just as condemned either way.
So this is only a question for women who believe in Christ and who are members of His body. Is there a problem in the Body of Christ for women to be topless?
Everything is permissible, but not everything is beneficial. Everything is permissible, but not everything edifies. -- 1 Corinthians 10
Although some translations attempt to place "everything is permissible" in the mouths of the Corinthians rather than Paul, the text does not make that indication (Koine Greek doesn't have quotation marks) and not all translations make that leap of judgement. More significantly, it fits with Paul's teaching about the Law in other places, such as Romans and Galatians, that Paul would have told the Corinthians that they were not bound by the Law--the written code.
In other words, the written code did not contain either a prescription or a proscription of specific activity that would lead to a righteous life. The Pharisees constantly examined the scriptures for acts that were sinful and were scrupulous in avoiding anything that might have been a sin. Yet, Jesus stated that righteousness demanded more than that. Righteousness is a higher bar than finding "sin" in the bible and avoiding it.
Whether Paul had originally said "everything is permissible" or the Corinthians coined the phrase themselves, Paul did not dispute that. Rather, Paul established two new standards of judging the actions of a Christian:
1. Choose actions that are beneficial.
2. Choose actions that edify.
Make no mistake, this is a far higher standard than "it says in the bible right here that is a sin." Do those things that are beneficial, do those things that edify.
Therefore let us not pass judgment on one another any longer, but rather decide never to put a stumbling block or hindrance in the way of a brother. I know and am persuaded in the Lord Jesus that nothing is unclean in itself, but it is unclean for anyone who thinks it unclean.
For if your brother is grieved by what you eat, you are no longer walking in love. By what you eat, do not destroy the one for whom Christ died. So do not let what you regard as good be spoken of as evil. For the kingdom of God is not a matter of eating and drinking but of righteousness and peace and joy in the Holy Spirit. Whoever thus serves Christ is acceptable to God and approved by men. So then let us pursue what makes for peace and for mutual edification.
Do not, for the sake of food, destroy the work of God. Everything is indeed clean, but it is wrong for anyone to make another stumble by what he eats. -- Romans 14
Although Paul is using food as his example because food was the particular issue at hand, this applies to everything else: Do not be the cause of a brother to stumble.
Everything is permissible.
We're not looking at the specific act for inherent good or bad, but rather how it affects our relationship, first, with other members of the Body of Christ, and secondly how it affects the mission of the Body of Christ in the world (1 Corinthians 5:1).
So let's say that Christian Fred likes to have a drink or two of wine at meals, have a pleasant conversation with his wife, and go to bed. There is no sin in Fred's drinking.
But Fred invites to dinner brother Melvin who he knows is a alcoholic, yet Fred pours Melvin a drink, and encourages Melvin to have a second, being aware that Melvin might then leave the meal, buy a bottle of wine, and end the night in a drunken stupor or worse. It doesn't matter whether the act itself is inherently a sin. It is not a sin for Fred, but it is a sin for Melvin, and Fred led Melvin to Melvin's sin.
It also doesn't make a difference whether the matter leads Melvin to the same action. Melvin may merely think, "Well, Fred takes liberties according to what he likes, so I will take liberties according to what I like," and Melvin may decide to go gambling--which is the liberty he craves. Fred has caused a brother to stumble, and that is wrong.