Green’s Dictionary of Slang

(US) a would-be cowboy who has the clothes but is otherwise spurious.

1926		D. Branch Cowboy and His Interpreters 17: The range came to expect and recognize the ‘mail-order cowboy,’ who arrived already fitted in cowboy-wear as he knew it from his reading and the assurances of some Middle Western store-keeper.
1936		R.F. Adams Cowboy Lingo 26: A tenderfoot in ‘custom-made’ cowboy regalia and devoid of range experience was a ‘mail-order cowboy’.