Women in Bangkok dress like Buhddist do for the most part because that is mostly what they are. That means they dress very modestly. They dress as they believe as per their religion. Women are supposed to completely cover their cleavage and their shoulders. They can wear pants or skirts but only those that are of at least knee-length. It is courteous and respectful to at least somewhat dress as the woman in the country do if you are a woman and will be visiting Bangkok.
It would be equally respectful if they do the same when visiting your country or at least not be more revealing than your culture allows. Being modest never hurts anyone and it can also be a sign that you are not interested in any extra activities. Wearing very skimpy clothes could indicate otherwise. Just as when in Rome, do as the Romans do, the same is true in Bankok, to an extent anyway.