An interesting hypothesis.Sorry, but this Jokes author is implying Blondes are dumb but could the author of this joke may be her intellectual equal.
But as a person of the blond type myself, I have a different take on the "story".
The tourists were from the USA and were being a bit loud and crass, as is sometimes the case with their fellow countrymen when travelling abroad.
The blonde waitress was student of mathematics at the nearby University of North Wales in Bangor. A bit of a Rachel Riley* sort of person, who was doing a part-time job to earn a few quid.
When asked the all important question:
"How do you pronounce:"
The devil made her give the facetious answer...
JimB
* https://en.wikipedia.org/wiki/Rachel_Riley