“Looks good & flies good” was printed in the Los Angeles (CA) Times in an aircraft classified ad on June 30, 1968. “The saying in World War II was that if it looks good it flies good” was printed in the Los Angeles (CA) Times on June 21, 1976. Authorship of the aviation saying is unknown.
 
CBS News “Man on the Moon:” The 50th anniversary of the Apollo 11 landing, on July 16, 2019, reported what astronaut Neil Armstrong said in 1969:
   
(Neil—ed.) Armstrong replied of the lunar module. “You know, there’s an old saying in aviation that ‘if it looks good it flies good.’ And this has to be the exception to the rule.  Because it flew very well. But it is the—probably the ugliest flying machine that was ever been designed.”
 
“If you look good, you play good” is a similar sports adage.
     
               
30 June 1968, Los Angeles (CA) Times, pg. 63, col. 2 classified ad:
AIRCRAFT
(...)
ERCOUPE. Looks good & flies good. $1500. Call ST0-9769
   
21 June 1976, Los Angeles (CA) Times, “SST ODYSSEY: Concorde: Quiet Luxury at Mach 2” by David Larsen, pt. 1, pg. 16, col. 1:
William E. Cole II, president of a printing press equipment firm in Seattle, stares at the curvature of the planet earth 11 miles below.
 
“The saying in World War II was that if it looks good it flies good, ” he said. “Well, this qualifies on both counts.”
     
Google Books
Aerospace Historian
Air Force Historical Foundation
Volumes 34-35
1987
Pg. 238:
The “old saw” that if it looks good, it flies good, certainly applies.
 
Google Books
Tracking Apollo to the Moon
By Hamish Lindsay
Singapore: Springer
2001
Pg. ?:
There’s an old saying about flying machines (grammatically questionable - but generally appropriate): ‘If it looks good, it flies good.’
     
27 October 2001, Fort Worth (TX) Star-Telegram, “Joint Strike Fighter,” pg. 18A, col. 1:
“There is an old saying, ‘If it looks good, it flies good.’”
—JSF design team member Ken Renshaw on the aircraft’s sleek appearance
 
Google Books
Superhawks:
Strike Force Bravo
By Mack Maloney
New York, NY: St. Martin’s Press
2004
Pg. ?:
There was an old saying in aeronautics: If it looks good, it flies good. But the reverse was also true
 
Google Groups: rec.aviation.military
my top 10 planes (+1)
Rickenbacker
10/25/96
(...)
As you might have noticed I go by the old creed, “If it looks good, it flies good” 😊.
 
Twitter
Paul “Woolly Rhinoceros” J
@maxcelcat
Replying to @falcon124
And you know what they say - if it looks good, it flies good. Will have to wait and see if it sells good and drinks petrol good!
12:08 AM · Dec 16, 2009
 
Twitter
wtuckwong
@wtuckwong
Replying to @DeathOverPain
@wtuckwong
Aerospace Technology. “If it looks good, it flies good.”.  ^^
11:27 AM · Jan 13, 2013
 
CBS News
“Man on the Moon:” The 50th anniversary of the Apollo 11 landing
JULY 16, 2019 / 11:02 PM / CBS NEWS
(...)
“Looks pretty much the same, yeah,” (Neil—ed.) Armstrong replied of the lunar module. “You know, there’s an old saying in aviation that ‘if it looks good it flies good.’ And this has to be the exception to the rule.  Because it flew very well. But it is the—probably the ugliest flying machine that was ever been designed.”
 
Twitter
Dave Hopple
@DaveHopple
Sad to see the Red Dragon leave the @jdcmotorsports shop and go to a new home across the pond.
I watched a documentary about the late Neil Armstrong last night and he said, “if it looks good, it flies good” I agree… https://www.instagram.com/p/B0BL-MTFwbL/?igshid=7fdpvtzrbmds …
8:52 AM - 17 Jul 2019