“I didn’t like her apartments, so I knocked her flat” is a joke that was printed in the El Paso (TX) Herald on July 17, 1920. “I didn’t like her apartment so I knocked her flat” was printed in The Mixer (Joliet Works Illinois Steel Company) in November 1920. The line probably comes from vaudeville.
 
“Louis A. Mayhew has just composed a popular song entitled ‘She Criticized My Apartment and I Knocked Her Flat’” was printed in the Winston-Salem (NC) Journal on February 8, 1921. This (“She criticized my apartment, so I knocked her flat”) is the usual form of the pun.
   
               
Chronicling America
17 July 1920, El Paso (TX) Herald, pg. 7, col. 4:
“I didn’t like her apartments, so I knocked her flat,” said the comedian telling of his call on his girl.
   
Google Books
November 1920, The Mixer (Joliet Works Illinois Steel Company), “Plant Gossip,” pg. 11:
I didn’t like her apartment so I knocked her flat.
 
8 February 1921, Winston-Salem (NC) Journal, “Kiwanis Club Will Entertain Newcomers,” pg. 2, col. 6:
Louis A. Mayhew has just composed a popular song entitled “She Criticized My Apartment and I Knocked Her Flat,” which he will sing during the evening.
     
Google Books
4 November 1921, The American Legion Weekly, “Bursts and Duds,” pg. 16, col. 3:
Heart Rent-ing.
“I hear you had a quarrel with your sweetheart the other day.”
“Yes, she sneered at my apartment so I knocked her flat.”
 
27 October 1974, Philadelphia (PA) Inquirer, “Folk Festival Has Grown Quickly But Well” by Steve Twomey, pg. 13, col. 6:
... John Prine and his friend, Steve Goodman, with their batch of songs that specialize in insane lyrics (“She criticized my apartment, so I knocked her flat”); ...
   
20 June 1981, The Globe and Mail (Toronto, ON), “Mermaid Inn: If it was good enough for the Bard” by Shirley Whittington, pg. 6, col. 3:
There is a kind of comic glory in quietly slipping “She criticized my apartment, so I knocked her flat” into a conversation.
   
Google Books
All Kinds of Humor:
Jokes, Quips, and Fun Stuff for Many Occasions, Book 2
By Frank Verano
Xlibris Corporation (Xlibris.com)
2012
Pg. 210:
She criticized my apartment, so I knocked her flat.
 
Reddit—Jokes
She criticized my apartment…
submitted December 30, 2014 by onlywookie
So I knocked her flat
 
Reddit—Oneliners
She criticised my apartment, so I knocked her flat.
submitted May 14, 2018 by Belmish
 
Reddit—Oneliners
She criticized my apartment, so I knocked her flat.
submitted April 14, 2018 by thats-fucked_up