“If it’s stupid but (it) works, it isn’t stupid” is one of “Murphy’s Law of Combat” that was printed by David Evans in the Chicago (IL) Tribune in 1989; Evans credited an unnamed army colonel as the author. The saying has been frequently reprinted in a longer and longer list of “Murphy’s Laws of Combat.”
 
 
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10 May 1989, Lewiston (ME) Journal, “New officers should inspect Murphy’s cribsheet” by David Evans, pg. 6A, col. 2:
3. If it’s stupid but works, it isn’t stupid.
(From an unnamed army colonel’s “Murphy’s Laws of Combat”—ed.)
 
28 June 1991, Galveston (TX) Daily News, “Dissolving family key to country’s woes” by Bob Dambach, pg. 6, cols. 2-3:
The ideas I am about to put forward will be called stupid by many who are more learned than I, but I’m applying Murphy’s Law of Combat Operations, “If it’s stupid but it works, it ain’t stupid.”
 
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More Than a Game:
The Computer Game as Fictional Form
By Barry Atkins
Manchester: Manchester University Press
2003
Pg. 68:
As it also says in the Opposing Force manual, ‘If it’s stupid but works, it isn’t stupid.’
 
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Too Stupid to Quit:
Banking and Business Lesson Learned the Hard Way
By Richard D. Jackson
Bloomington, IN: iUniverse
2005
Pg. 128:
The old adage someone coined, probably in a ridiculous situation, sums up our attitude about many of the things we tried at First Georgia and Georgia Federal. “If it’s stupid but works, it ain’t stupid.”
 
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1951:
It Ain’t Bragging If You Can Do It
By David D. Ferman
Bloomington, IN: AuthorHouse
2012
Pg. 143:
Like they say, “If it’s stupid but it works, it ain’t stupid.”