"着朴素的服装" can be translated into English as "wear plain/simple clothes" or "dress in modest attire".
"Wear plain/simple clothes" emphasizes the simplicity and lack of ornamentation in the clothing.
"Dress in modest attire" conveys a sense of humility and restraint in one's appearance, which may also align with the idea of "朴素" (pǔ sù) in Chinese.
Both expressions are appropriate depending on the context and the desired nuance.