“If you can’t say something nice, don’t say nothing at all” is advice given to the character Thumper in the Disney animated movie Bambi (1942). The proverbial saying is older than the movie, however.
 
“If you can’t say something nice, become a reporter” is a jocular anti-proverb that has been cited in print since at least 2001. The authorship of the humorous saying is unknown.
 
   
Wikipedia: Thumper (Bambi)
The character Thumper first appears in the film Bambi, watching as Bambi is first presented as the young prince to the creatures of the forest. He remarks that Bambi is “kinda wobbly” but is reproved by his mother who makes him repeat what his father had impressed upon him that morning, “If you can’t say something nice, don’t say nothing at all”. This moral is now known by such names as the “Thumperian principle”, “Thumper’s rule” or “Thumper’s law”.
   
Google Books
The Funniest Jokes in the World
BY H. D. Shourie
New York, NY: Penguin Books
2001
Pg. 49:
If you can’t say something nice become a reporter.
 
Google Books
A Heart’s Refuge
By Carolyne Aarsen
New York, NY: Steeple Hill Books
2004
Pg. 286:
“Well, you know the saying, if you can’t say something nice, become a reporter.” He couldn’t stop his burst of laughter.
   
Funny Buburuza
October 24th, 2008
If you can’t say something nice, become a reporter.
Unknown
   
Friendship
If you… #2
by on SEPTEMBER 18, 2010
(...)
“If you can’t say something nice, become a reporter.”
   
My Time Matters Blog
One Liners – I
January 15, 2011 By Ace Lukman
(...)
If you can’t say something nice, become a reporter.