Apart from on the beach, wearing shorts, swimwear and skimpy clothing including sleeveless t-shirts is considered improper and low-class attire. It is acceptable for children. Regardless of the heat, you should wear long trousers in town – and actually, wearing light loose clothing does help keep you cool, as well as complying with Thai standards of modesty.
Going nude or topless on beaches is considered unacceptable and offensive (a
lot of Thais swim fully clothed) and in some cases is illegal.