The New York Mets became a baseball expansion team in the early 1960s and proceeded to lose many, many games. However, New York fans still attended games to cheer them on. “Lovable Losers—Next, the Mets?” by Bill Prochnau in the Seattle (WA) Times was published in April 20, 1962, comparing the old Brooklyn Dodgers to the new New York Mets. “The Mets: Lovable Losers” by Jimmy Breslin was published in the Chicago (IL) Tribune on May 12, 1963. The Mets shed the “lovable loser” tag when they won the World Series in 1969.
 
Many other teams acquired the name “lovable losers” in the 1970s, most notably the Chicago Cubs. “Chicagoans long will remember the year their lovable losers defied the oddsmakers and made their pennant runs”—referring to both Chicago baseball teams—was published by UPI in October 1977. “Indeed, Reuschel clearly has no patience with attempts to label the Cubs ‘lovable losers’ on a par with the 1962 Mets” was published in the Chicago (IL) Tribune on May 27, 1981.
 
     
Wikipedia: New York Mets 
The New York Mets are an American professional baseball team based in the New York City borough of Queens. The Mets compete in Major League Baseball (MLB) as a member club of the National League (NL) East division. The Mets are one of two Major League clubs based in New York City; the other is the New York Yankees.
 
Wikipedia: Chicago Cubs
The Chicago Cubs are an American professional baseball team based in Chicago, Illinois. The Cubs compete in Major League Baseball (MLB) as a member club of the National League (NL) Central division. The team plays its home games at Wrigley Field, located on the city’s North Side. The Cubs are one of two Major League teams in Chicago; the other, the Chicago White Sox, are members of the American League (AL) Central division.
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Other nicknames
The Cubbies, the North Siders, the North Side Nine, the Boys in Blue, the Lovable Losers, the Little Bears, the Blue Bears, the Baby Bears
 
20 April 1962, Seattle (WA) Times, “Lovable Losers—Next, the Mets?” by Bill Prochnau, pg. 15, col. 1:
That’s the way they were in Brooklyn—lovable losers.
 
12 May 1963, Chicago (IL) Tribune, “Just Browsing,” sec. 1, pg. 20, col. 4:
The Mets: Lovable Losers
By Jimmy Breslin
 
21 May 1963, Democrat and Chronicle (Rochester, NY), “Marvelous Marv” by George Beahon, pg.D1, col. 2:
Because the Mets are back in 10th place again. So you see, they’ll be needing Marvelous Marv tp help recapture the image of the lovable losers.
 
7 July 1963, New York (NY) Times, “An Image in Concrete: This Is Shea Stadium. The Mets May Play in It Some Day” by Robert Lipsyte, pg. S3, col. 5:
And the Mets can relax, too, as loose and lovable losers. For a while.
 
9 August 1966, Washington (DC) Post, “Mets Provide Happiness With an ‘Old Pro’ Team” (AP), pg. C2, col. 1:
For almost four years, Stengel’s oratorial excellence and the enchanting blunders of players like Throneberry, helped make the Mets baseball’s most lovable losers.
   
11 September 1969, Austin (TX) Statesman, pg. A17, col. 4:
Those Mets:
‘Lovable Losers’ Leading NL Division
   
25 September 1969, Austin (TX) Statesman, pg. A15, col. 3:
Laughable, Lovable Losers Finally Win
   
Google Books
Amazing Baseball Teams
By Dave Wolf
New York, NY: Random House
1970
Pg. 150:
They would chant in unison “Let’s Go Mets” in the final inning, even if the team was losing by ten runs — which it often was. While the Mets kept on dropping game after game, the fans kept pouring into the ballpark to root for the lovable losers.
 
6 October 1977, Richmond (VA) Times-Dispatch, “Chicago Will Recall 1977” (UPI), pg. F-4, col. 1:
Chicagoans long will remember the year their lovable losers defied the oddsmakers and made their pennant runs.
 
27 May 1981, Chicago (IL) Tribune, “Reuschel accepts his fate with Cubs” by Bob Logan, pg. C2, col. 1:
Indeed, Reuschel clearly has no patience with attempts to label the Cubs “lovable losers” on a par with the 1962 Mets, the all-time clowns who lost 120 games.
 
21 February 1982, Chicago (IL) Tribune, magazine, pg. 8:
Can the Cubs be saved?:
New management puts the squeeze on baseball’s lovable losers.
By Skip Myslenski
     
Bleacher Report—Chicago Cubs
Lovable Losers? Not So Much: Why Everyone Hates the Chicago Cubs
By Kevin Trahan , Analyst Jan 18, 2009    
They’re called the lovable losers and will forever live in infamy for their losing streak of 100 years and counting. Cubs fans have become accustomed to chanting “maybe next year” and cheering their team into the playoffs, only to have their hopes smashed.
 
OCLC WorldCat record
The legend of Zippy Chippy : life lessons from horse racing’s most lovable loser
Author: William J Thomas
Publisher: [Toronto] : McClelland & Stewart, [2015]
Edition/Format:   Print book : EnglishView all editions and formats
Database: WorldCat
Summary:
From acclaimed humorist William Thomas comes the funny yet poignant story of a thoroughbred racehorse that lost 100 races in a row—but, in everyone’s eyes, became the ultimate winner.—
   
Today’s Knuckleball
Heyman: 7 Cubs moves that turned Lovable Losers into winners
By Jon Heyman
Posted on September 22, 2016