“Never trust a skinny chef” is an old saying. The implication is that the chef doesn’t want to eat the food that he makes. “Don’t/Never judge a book by its cover” is a popular idiom. “Never judge a cook by his blubber” is a saying that references these two other sayings.
   
“I used to think that fatsos were the best cooks… then I met a great skinny cook. Just as you can’t judge a book by its cover, you can’t judge a cook by his blubber” was posted on the newsgroup rec.food.cooking on March 24, 2003. “Don’t judge a cook by his blubber” was posted on Twitter by Gregg Weiss on July 20, 2010. “Never judge a cook by his blubber. #newoldsayings” was posted on Twitter by Eddie White on September 6, 2010.
   
“I used to assume that if a chef is fat, it means that their food is good. Then I learned: Never judge a cook by his blubber” was posted on Reddit—Jokes on January 9, 2021.
   
     
Wikipedia: Don’t judge a book by its cover
The English idiom “don’t judge a book by its cover” is a metaphorical phrase which means “you shouldn’t prejudge the worth or value of something, by its outward appearance alone”.
   
Early reference
. In George Eliot’s The Mill on the Floss (1860), Mr Tulliver uses the phrase in discussing Daniel Defoe’s The History of the Devil, saying how it was beautifully bound.
. The preceding version was then publicised by the 1946 murder mystery novel by Edwin Rolfe and Lester Fuller, Murder in the Glass Room, in the form of “You can never tell a book by its cover.”
   
Google Groups: rec.food.cooking
Question?
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Mar 24, 2003, 7:20:08 PM
I used to think that fatsos were the best cooks… then I met a great skinny cook.
Just as you can’t judge a book by its cover, you can’t judge a cook by his blubber.
     
Google Groups: rec.food.cooking
Fat waitstaff
.(JavaScript must be enabled to view this email address)
May 10, 2003, 11:30:04 PM
Johnny wrote:
>I was taught to never trust a thin chef or
>thin restaurant manager/owner. I haven’t
>witnessed too many fat, friendly
>waitresses. My experience has been that
>fat waitresses are always complaining
>about something.
 
You can’t judge a cook by his blubber.
         
Twitter
Gregg Weiss
@greggweiss
Don’t judge a cook by his blubber.
4:45 PM · Jul 20, 2010·Twitter for iPhone
   
Twitter
Eddie White
@mrwhitepaisley
Never judge a cook by his blubber. #newoldsayings
3:35 AM · Sep 6, 2010·Echofon
 
Bleacher Report—MMA
Fedor Emelianenko, Josh Barnett and the Pudgiest Fighters in MMA Today
JONATHAN SHRAGER
JULY 27, 2011
(...)
But whilst the lean, mean fighting machine may have duped us in the past, us seasoned MMA aficionados know better than to always judge a book by its cover, as you might judge a cook by his blubber.
   
Twitter
Church Curmudgeon
@ChrchCurmudgeon
Replying to @ChrchCurmudgeon
Never judge a cook by his blubber.
10:49 PM · Mar 19, 2018·Twitter for iPad
 
Twitter
Lilly Winwood
@LillyWinwood
Don’t judge a cook by his blubber
3:57 PM · May 27, 2018·Twitter for iPhone
 
Reddit—Jokes
Posted by u/Moerox111 January 9, 2021
I used to assume that if a chef is fat, it means that their food is good.
Then I learned: Never judge a cook by his blubber.