”Dingle dårhus” is not a wellknown expression in Sweden.The references on Google might well originate from the film/book by Nesser. But there is a small community in the southwest of Sweden, north of Gothenburg, called Dingle. (Pronounced Ding – le) and there seems to have been an asylum there, ackording to Google.
A personal comment: Dingle dårhus is mentioned in connection with the mad aunt, who ran into a moose with her bike, and then spent the rest of her life in an asylum.
Dingle is not a ”typical” Swedish word, it almost sounds like dingla (English dangle). It's hard to believe that the ”moose-episode” made the poor aunt mad. But since mental disorders were something embarrising, the children might have been told the sad story in a more fun way, with the moose, and the name Dingle might have added more absurdity to it. And the alliteration might have made it easier to remember. That's the feeling I get.