It was not an insult.
The "Pizza Americana" does typically come topped with french fries and hot dogs although it can also have other toppings.
It is usually ordered by children and not adults.
If they were, in fact, smirking, it was probably because the Americana name pokes fun at our dietary habits and the fact that some Americans came into their pizzeria and actually ordered it was probably pretty funny to them.
Had I been in the pizzeria and watched you order it, I would have been amused also.
They did not make you some bespoke, disgusting pizza that had nothing to do with what that item was on the menu.
As to Italians' treatment of Americans, perhaps things have changed since the election in 2024, but I doubt it.
We have been up in Canada a few times and there is no anti-Americansim that I can detect.
In fact Italians tend to like Americans much better than the French, Germans or the English. Although that probably has more to do with the fact that we are more generous and spend more money than those other nationalities.
I do think Italians, and they are not alone in this regard, have a sense of tourist-fatigue as Italy is now overrun with tourists all year long.
We will be there for 3 weeks in late August/September and that will give me a good sense of whether something has changaed.