So was that a joke? Iowa confused me too. Out of nowhere question.
The joke is that everything is EXTRA YUGE in America (especially from an Asian PoV), with Iowa snubbing her room accommodations despite them being considerably lavish and expansive (hotel suite like, basically) compared to the average Japanese dorm.
Of course she also did said snubbing with a 'joke' of her own (i.e. "it's so cramped I don't even have room to brush my teeth, so I'll need to see a dentist one day").
The comic vastly exaggerates this of course, for 'comedic' effect, though there is some real life basis in the sense of some actual Americans having issues with how relatively cramped Japanese rooms can be, as related by anecdotal experiences on both sides.
Is there a good dentist around here?I'll be getting more cavities, seeing as...This is your room, Iowa-san.DentistI can't even brush my teeth in this closet.I wonder if this is what they call an American-style joke.I hope it's not too cramped compared to what you're used to.A dentist?Have you... got a toothache?