Mail-Order Cowboy

A “mail-order cowboy” was a person newly arrived to the west, dressed as if from a mail-order cowboy catalogue. Other names for the “mail-order cowboy” include Arbuckle” and tenderfoot.”
 
The term “mail-order cowboy” was added to cowboy lingo late—after 1900.
 
 
The Wild West
MAIL-ORDER COWBOY
This was a derogatory term used to chide tenderfoot, urban “cowboys” who arrived from the East all decked out in fancy but hardly practical Western garb.
 
Google Books
Cowboy Lingo
by Ramon F. Adams
New York: Houghton Mifflin Company
2000 (reprint)
1936 (original copyright)
Pg. 26:
A tenderfoot in the “custom-made” cowboy regalia and devoid of range experience was a “mail-order cowboy.”

(Historical Dictionary of American Slang)
mail-order adj. ridiculously inadequate or inferior [Early quots. ref. chiefly to WWI.]
(...)
1926 Branch Cowboy 17: The range came to expect and recognize the “mail-order cowboy,” who arrived already fitted in cowboy-wear as he knew it.
 
29 November 1929, Helena (MT) Independent, “Is a Tenderfoot a Dude?,” pg. 4, col. 2:
The Indians express their impression of them in the term “mail-order cowboy.”