It doesn't matter what strata of society one comes from, Thais are very concerned about their appearance. Don't go thinking that the casual attire you see one the islands and beaches is the norm in Thailand. Thais are very concerned to look good and tend to dress a little more conservatively than your average Westerner. Just a trip to the local shopping centre, or even the supermarket will see Thais make sure they are "politely dressed" with men usually in long pants or jeans and women wearing something decent. Women always make a point of making sure that their hair is done and their make up is applied. It is not really that common for Thais to go to any sort of public place apart from parks, beaches or sports facilities looking casual. Judgment is made on one's appearance very quickly in Thailand and Thais are conscious of this and do their best to look decent.