I'm British, and heard jibes on Frasier (they all laughed) and I'm sure other US shows?
What's so unusual about the place or people? Maybe it's just by chance and the ''joke'' was simply context relating to the characters?
Every place is slightly different from other locations in the US and those differences are what makes up the joke. Washington state has Starbucks, grunge rock, hiking boot type fashion, etc.
https://www.movoto.com/guide/spokane-wa/spokane-stereotypes/
https://theculturetrip.com/north-america/usa/washington/articles/11-stereotypes-seattleites-wont-even-try-to-deny/
Spokane is a town in the eastern part Washington state which is generally the more rural area of Washington.
Frasier takes place in Seattle, which is the most cosmopolitan city in Washington in the west part of the state with other urban areas.
It would make sense that Frasier and his snob brother would mock the more rural and "uncultured" town of Spokane because they are elites and look down on small town folks.