Mud has been called a “great leveller” because a muddy playing field makes it difficult to run up and down, thus preventing much scoring. “Mud, like death, is a great leveller” was cited in a 1914 New Zealand newspaper, in a story about rugby football. “Mud, the great leveler” was cited in 1930 and “Mud being the great leveller” was cited in a 1959 article, both about American college football games. “Mud is supposed to be the great leveller at the races” was cited in a 1964 newspaper article about a Florida horse race.
 
   
Papers Past
14 August 1914, Ashburton (New Zealand) Guardian, “RUGBY FOOTBALL—Ashburton Competitions,” pg. 3, col. 3:
Mud, like death, is a great leveller, to the Southern Cross and High School teams found at the Domain yesterday. Southern Gross won the scramble by nine points to three.
 
26 November 1930, Cleveland (OH) Plain Dealer, “Mud and Glory: An Inside Story of Football” by James Neville, pg. 21 col. 1:
The play was not spectacular. Mud, the great leveler, had stopped all that.
 
Google Books
6 November 1959, Princeton Alumni Weekly, “Princeton 7, Brown 0” by Donald C. Stuart Jr., pg. 17, col. 2:
Mud being the great leveller, it was no particular surprise that the slow, slippery going saw Princeton barely eke out enough points to step into the first-place tie offered it when Penn and Yale were upset.
     
Google News Archive
4 January 1964, Palm Beach (FL) Daily News, “Sellers Rides In Handicap At Tropical,” pg. 5, col. 1:
Mud is supposed to be the great leveller at the races. But on Monday at Tropical Park, it was just the opposite. The winning margins were among the longest of the meeting.
 
Google Books
The Goalkeeper’s History of Britain
By Peter Chapman
London: Fourth Estate
1999
Pg. ?:
Mud was the ‘great leveller’, especially for a keeper. It bogged the others down, evened out the odds.
 
OCLC WorldCat record
Mud : the great leveller : destroyer of the best laid plans.
Author: Cromwell Productions.; Pegasus Entertainment.
Publisher: [Aldershot] : Pegasus Entertainment, ©2001.
Series: The War file
Edition/Format: DVD video : English
 
Google Books
The Boys’ Book of Soccer 1966
By Dennis Smith
London : Evans
2007
Pg. 25:
Mud is a great leveller, it is said.
   
The Southland Times (New Zealand)
Physical, even battle in heavy conditions ahead  
CLUB RUGBY: Division 2
DON WRIGHT  
Last updated 05:00 25/07/2013
It has been said that if rugby in Southland was ever to die, two of its last resting places would be Tokanui and the Coalfields.
(...)
The old adage that mud is a great leveller came through in three of the four quarterfinals.