“Sharp as a marshmallow sandwich”

A marshmallow—especially a toasted marshmallow—is not sharp. “I’m as sharp as a marshmallow tonight” was cited in print in 1946.
 
“Like I said, he’s about as sharp as a marshmallow sandwich” was posted on the newsgroup alt.sports.football.pro.pitt-steelers on July 23, 1997. “Bush is about as sharp as a marshmallow in a microwave” was posted on Twitter on September 29, 2013.
 
     
Google Books
Too Sweet for Sixteen;
A Comedy of Youth
By James Floyd Stone
Franklin, OH: Eldridge Entertainment House
1946
Pg. 29:
SHRIMP (sits on sofa; laughs]: Well, shoot it to me. I’m as sharp as a marshmallow tonight.
   
Google Books
Astounding Science Fiction
Volume 38, Issue 4
1947
Pg. 24:
“Sharp as a marshmallow, and disgustingly productive. Will you come over?”
 
11 December 1958, Philadelphia (PA) Inquirer, “Screening TV” by Harry Harris, pg. 29, col. 1:
Culminating in a knock down, drag out fight between a Hungarian glamor girl and a five-by-five gal of much earlier vintage, it was about as sharp as a marshmallow.
       
Google Groups: rec.humor
“Not all there” terms, Unfinished list of
MattHub42
6/26/97
How about this one
“He/she is as sharp as a marshmallow”
 
Google Groups: alt.sports.football.pro.pitt-steelers
O’Donnell or Bettis?
DJC
7/23/97
(...)
Like I said, he’s about as sharp as a marshmallow sandwich
 
Twitter
GammaRaeResistance‏
@GammaRae206
Replying to @AustinKwikkel
@AustinKwikkel Not much, obviously. Bush is about as sharp as a marshmallow in a microwave. Crawford’s very own unelected Village Idiot.
10:39 PM - 29 Sep 2013
 
A Way with Words
Marshmallow Sandwich
Posted by grantbarrett on October 25, 2014
Someone on the dull side might be described as “sharp as a marshmallow sandwich.” This is part of a complete episode.
 
Google Books
Petlandia
By Peter Hannan
New York, NY: Scholastic Press
2015
Pg. 109:
“He’s as sharp as a marshmallow, and I’m talking about a melted marshmallow ...”
 
Twitter
GammaRaeResistance‏
@GammaRae206
Replying to @drskyskull
Sane Iowans (and others) know .@SteveKingIA is as much of a mental midget as Donald Dunce. Sharp as a marshmallow in a microwave
10:55 PM - 5 Mar 2017 from Mankato, MN