“Too much attention to the pigskin doesn’t help the sheepskin” is a popular saying; “sheepskin” means a college academic diploma and “pigskin” means the covering of a football, symbolic of playing sports. The saying has been cited in print since at least November 1923 (when it was credited to the Lake County Times) and December 1923 (when it was credited to the Detroit News).
 
“Pigskin” (presenting sports) and “sheepskin” (representing academics) have been compared since at 1893. In 1895, the Boston (MA) Transcript said that two skin games were played at Harvard—sheepskin and pigskin.
 
 
18 November 1893, Weekly Wisconsin (Milwaukee, WI), “The Pursuit of the Pigskin,” pg. 4, col. 2:
Pigskin is the material of the cover of the football which is kicked by the college young man.
 
Time was when the youth of the land went to college in quest of a sheepskin; but the pursuit of the pigskin is the engrossing object of the majority nowadays, and the sheepskin takes a secondary place. Is this wise?
 
12 January 1895, The Centralie Enterprise and Tribune (Centralia, WI), pg. 4 (?), col. 4:
Harvard’s Skin Game.
The Boston Transcript says that two skin games are played at Harvard—namely, sheepskin and pigskin.
 
16 January 1917, State Times (Baton Rouge, LA),  “Luke M’Luke Says,” pg. 4, col. 4:
The old fashioned youth who used to go to college to get fame with the sheepskin now has a son who goes to college to get fame with a pigskin.
 
25 November 1923, Sunday Times-Advertiser and Trenton Evening Times (Trenton, NJ), pg. 6, col. 4:
OR THE CUTICLE.
Too much attention to the pigskin doesn’t help the sheepskin.—Lake County Times.
 
5 December 1923, Ukiah (CA) Republican Press, pg. 3, col. 3:
Too much attention to the pigskin doesn’t help the sheepskin.—Detroit News.
 
22 March 1936, Los Angeles (CA) Times, “Pen Points,” pg. A6:
Many a college student has failed because he paid too much attention to the pigskin and not enough to the sheepskin.
 
Google Books
Speaker’s Handbook of Epigrams and Witticisms
By Herbert V. Prochnow
New York, NY: Harper
1955
Pg. 111:
Too much attention to the pigskin doesn’t help the sheepskin.
Detroit News
   
Google Books
A Dictionary of American Proverbs
Edited by Wolfgang Mieder, Stewart A. Kingsbury and Kelsie B. Harder
New York, NY: Oxford University Press
1991
Pg. 127:
Too much attention to the pigskin doesn’t help the sheepskin. Rec. dist.: Mich., N.Y.