Ketchup enhances most things.....

It's actually pretty good. Europeans do it rather than ketchup, at least they did in 1970-71.
Mayo on fries is like warm potato salad. German in origin and adopted across Europe particularly in Belgium where potato fries were first created.

It is also a very popular way for people in the Southern US to eat fries.

I prefer fries with mayo over any other condiment.

I always say, don’t knock it unless you have tried it.

A lot of opinions here seem to be judge first before trying. I like to do the opposite, try first and then decide if I like it...and don’t judge others that have a different opinion AFTER they have tried it.