In the Netherlands many/most people have mayo on their chips/fries, including me
But you do have to get the right one. Sometimes if you buy chips/fries at the so called 'snackbar' the mayonaise ruines the taste of the chips and sometimes if the chips aren't so tasty some mayonaise can make it very tasty.
But I heared that in teh USA having mayo on the fries is very not done *remembers a scene from a John Travolta movie**forgot the name*
and I rather have mayo than Mustard!