I am an American and I know "thick as a brick" means "very stupid," but I'm not sure where I learned it. I have heard it spoken here, but I read more than a book a day and have lots of British friends...Maybe most American teens wouldn't know it but educated oldsters ought to:-) And I think Duo ought to accept literal translations.
This is an idiomatic expression (all languages have them) and they are never translated literally. With that said, "dumb as a bread" carries no meaning in English. There are several equivalents in English that can reflect the meaning of this Norwegian phrase and Duolingo chose "thick as a brick".
But I don't see why this should be considered wrong per se. Just because the sentence doesn't make sense literally doesn't mean it shouldn't be a valid translation, especially for people who have no idea if the sentence is meant to be idiomatic or just weird (have you seen the Danish course, for example?)
That depends on whether you conceptualize Duo as a translation course, or a language course. "Dumb as a bread" isn't valid as an idiomatic translation; but it's valid as proof that the language student understood the Norwegian idiom. Indeed, one may argue that if you just memorize an equivalent idiom to the entire phrase, you're showing <sub>less</sub> Norwegian than someone who can demonstrate its working parts.