I went to school in Germany for a while back at the end of the 80's and mayo and fries was normal. I got very odd looks back in the UK asking for it for a long long time. The same sort of looks i get here (Australia) when i ask for chips without salt! Then I'll add insult to injury and all for mayo (UK style, not sweetened! )...
Mayo on my fries/chips was something I picked up completely on my own as a child. I absolutely love mayonnaise and can (and sometimes do) eat it straight from the jar, so dipping a bit of fried potato in some didn't seem unusual at all to me.
At home, the most I'd get would be, "I declare, that boy'd put mayonnaise on a rock and eat it!"
At school, it was a bit more difficult, because we didn't have mayo packets, so I'd have to ask one of the school cafeteria workers for a little bit on the side, and the answer was, "Move along, you little idiot!" - unless it was fish sticks or fish sandwich day, then I'd just ask for extra tartar sauce, and that was fine.
We have a very popular local chain restaurant here that's famous for, among other things, their fish sandwiches, which in turn means they're famous for their tartar sauce, because they also use it on their burgers, and so tartar sauce on fries is very much a thing here, especially at that restaurant.
