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.