A person who accepts a loss without whining (such as complaining about the officials) is said to be “a good sport.” A good sport says nice and gracious comments about the winner.
 
“The trouble with being a good sport is that you have to lose in order to prove it” is a joke that appeared in The Reader’s Digest in January 1952 and is still told.
     
 
The Free Dictionary
(a) good sport
someone who can accept a loss in a competition or can accept being the butt of a joke.
Bob is usually a good sport, but this time he didn’t seem to appreciate your joke.
 
8 July 1924, Manitowoc (WI) Herald-News, pg. 3, col. 3:
The trouble with being a good sport is that it takes too much time and pays such small returns.
 
13 January 1952, Van Nuys (CA) News, “News-Graphs” by Walter Mendenhall, pg. 1, col. 1:
YEZZIR
“The trouble with being a good sport is that you have to lose in order to prove it.”
—The Reader’s Digest
 
Google News Archive
7 February 1952, The Spokesman-Review (Spokane, WA), “Shop with Sue,” pg. 8, col. 8:
The trouble with being a good sport is that you have to lose to prove it.
 
27 September 1956, Springfield (MA) Union, “Walter Winchell On Broadway,” pg. 15, col. 2:
Morey Amsterdam’s logic: “The only trouble with being a Good Sport is—You have to Lose to be One.”
 
Google News Archive
3 October 1961, Meriden (CT) Record, “Today’s Chuckle,” pg. 1, col. 3:
The only trouble with being a good sport is that you have to lose to prove it.
   
Google Books
January 1977, The Rotarian, pg. 56, col. 1:
The only trouble with being a good sport is that you have to lose to prove it.
—The Iron Coast
WAIUKU, NEW ZEALAND
 
Google Books
Business Wit & Wisdom
By Richard S. Zera
Washington, DC: Beard Books
2005
Pg. 248:
The only problem with being a good sport is that you have to lose to prove it.