It means don't wear uniforms, that is, don't wear professional clothes. Wear something casual. "No" means don't. Standard clothing is a proper noun and a uniform type of clothing.

Standard clothing: refers to the clothing worn by a group of people in the same group, which is used to identify members who often wear uniforms in various occupations or different groups, such as students, soldiers, doctors, nurses, policemen, etc. ?

The office requires employees to wear suits and pants, which is called standard clothing.

The school uniforms that some schools require students to wear are also called standard uniforms.