I dunno...something tells me he'd be a lot more obscure. Perhaps his works, in translation, would be popular among the PBS-NPR-progressive type of parents for their kids but on the whole, you'd have to look long and hard for copies of his books (forget Barnes and Noble; only independent bookstores, mostly in university towns, would carry them--and a small assortment at most). And forget movies.
While we're at it, don't forget Astrid Lindgren's stuff doesn't quite make the cross-cultural translation from Sweden in particular / Europe in general to the US; there are a few things that don't seem quite right IIRC. Dahl would probably suffer from a similar not-quite translation--that is, if he doesn't already.