“Potatoes are a weird food like that, for example, many people put ketchup on french fries, but few people put ketchup on mashed potatoes” was posted on the newsgroup alt.music.smash-pumpkins on February 22, 1999. “Putting ketchup on french fries is OK; putting ketchup on mashed potatoes is not OK. #momlogic” was posted on Twitter  by ●ᴥ● on February 23, 2010. “we put ketchup on french fries, jo-jo’s, tater tots, home fries, and hasbrowns… why not mashed potatoes?” was posted on Twitter by Justin on October 11, 2010. “Why do we put ketchup on French Fries, but not baked/mashed potatoes?” was posted on Reddit—Showerthoughts on April 29, 2015.
 
Mashed potatoes are usually served with gravy, but almost never ketchup. However, most restaurants have a bottle of ketchup on the table, and diners can add ketchup whenever they want. Gravy is also sometimes put on french fries, in dishes such as poutine.
 
There are several possible explanations. Mashed potatoes is often served with turkey or steaks, and these dishes go better with gravy than with the sweetness of ketchup. Fried potatoes also hold ketchup better than mashed potatoes.
 
         
Wikipedia: Mashed potato
Mashed potato (British English) or mashed potatoes (American English and Canadian English), colloquially known as mash (British English), is a dish of mashing boiled potatoes, usually with added milk, butter, salt and pepper. It is generally served as a side dish to meat or vegetables. When the potatoes are only roughly mashed, they are sometimes called smashed potatoes.
(...)
Mashed potatoes may be eaten with gravy, typically meat gravy, though vegetable gravy is becoming more common as the vegetarian and vegan trends see a rise in popularity.
       
Google Groups: alt.music.smash-pumpkins
Mashed Potatoes: what is it?
Sean Duffy
Feb 22, 1999, 3:00:00 AM
(...)
You know, I don’t think I’ve ever had mashed potatoes with sour cream. I put sour cream on baked potatoes all the time, but never mashed potatoes. Potatoes are a weird food like that, for example, many people put ketchup on french fries, but few people put ketchup on mashed potatoes.
 
Twitter 
●ᴥ●
@dougoftheabaci
Putting ketchup on french fries is OK; putting ketchup on mashed potatoes is not OK. #momlogic
6:53 PM · Feb 23, 2010·Twitter Web Client
   
Twitter
Justin
@OnAirJustin
we put ketchup on french fries, jo-jo’s, tater tots, home fries, and hasbrowns… why not mashed potatoes?
4:43 PM · Oct 11, 2010·Twitter Web Client
   
Twitter
🤴🏾KING 💔
@StackBoyTay
#iGotQuestions WHY DO WE PUT KETCHUP ON FRIES BUT DON’T THINK TO PUT IT IN MASHED POTATOES??
9:25 PM · Aug 4, 2011·Plume for Android
 
Reddit—Showerthoughts
Posted by u/GothicWhoovian April 29, 2015
Why do we put ketchup on French Fries, but not baked/mashed potatoes?
     
Twitter
🌵Ascot @ emote catchup
@ascothero
How come we put ketchup on fries but not mashed potato? 🤔
10:52 PM · Sep 30, 2017·Twitter for Android
 
Reddit—Showerthoughts
Posted by u/Ryuk3112 April 26, 2018
The way it’s fundamentaly unacceptable to put ketchup on mash potato but as soon as you fry the potato it’s perfectly fine
 
Twitter
Goofbag
@punisher766
So people put ketchup on fries, hash browns, etc. Do any of you put ketchup on mashed potatoes?
3:12 PM · May 16, 2019·Twitter for Android
 
Twitter
Stanley Broniszewski
@StanleyBronisz2
Replying to @News12LI
Why is it that people put ketchup on french fries, which are made from potatoes, but never put ketchup on mashed potatoes?
9:25 PM · Jul 14, 2019·Twitter for Android
         
Reddit—ExplainLikeImCalvin
Posted by u/actuallyboa February 20, 2020
Hey Dad, why don’t people put ketchup on mashed potatoes and gravy on fries?
COMMENTS
[deleted]
This is actually quite simple, Calvin. Gravy is made by taking parts of a turkey you normally might not eat, and grinding them into a pulp. Thus, it’s suitable for plain mashed foods: things like potatoes, burgers, and cayenne pepper.
Ketchup is ground tomatoes full of all sorts of strange processes and additives they’d rather not put on the ingredients list, and is best suited for similar foods: things like french fries, candy, and American peanut butter.
Digituesday
Actually, people do put gravy on fries - or, at least, Canadians do. Gravy on fries (and cheese curds) is a dish called ‘poutine’. Some American restaurants that want to seem trendy also have begun serving poutine recently.
Similarly, there’s an old Southern american mashed potato recipe with ketchup, pepper and fried egg called ‘Bloody Mush’. Some folks think that it originally started in England, but it’s actually a dish that has its roots in the era of the Great Depression when poor folk didn’t have much and resorted to odd flavor combinations to keep their meals interesting. Bloody mush is still served at family gatherings and church socials to this day, however, in certain parts of the South.
 
Reddit—Showerthoughts
Posted by u/DelAguila182 September 16, 2020
Putting Ketchup on french fries is great. Putting Ketchup on mashed potatoes is madness.
 
Reddit—Showerthoughts
Posted by u/RZAtheAbbot January 9, 2021
People put ketchup on fries, roasted potatoes, hash browns but not on mashed potatoes.
 
Reddit—Showerthoughts
Posted by u/LokiiVegas January 10, 2021
it is perfectly fine to eat fries with ketchup but an absolute hate crime to put ketchup on mashed potatoes