Clothes make the man. Naked people have little or no influence on society.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Quoted in Merle Johnson, ed., More Maxims of Mark (1927)
    (Source)

Not found in a primary source. Johnson's 1927 work is a 15-page pamphlet, generally considered authentic.

More discussion of the quote and its origins: Quote Origin: Clothes Make the Man. Naked People Have Little or No Influence in Society – Quote Investigator®.

Twain is riffing off of a phrase ("clothes make the man") found in ancient Greek and Roman proverbs, including the Latin vestis virum facit, which passed into 15th Century English, and even shows up in Shakespeare. More information here: The Fascinating Origin of “Clothes Make the Man” | by Alexander Atkins | Medium.

See also this associated Twain quote.