Translation:You are not so bad, did you know that?
Yes, it's idiomatic. Saying that something, or someone, is "ikke så verst" - not too bad - is a compliment in Norwegian.
Saying that something is "ikke så dårlig", typically stressing "så", reads more like "not -that- bad", as in it's bad but could be worse; it's not quite as bad as someone makes it out to be.
Det is not the third person singular pronoun (it) in this sentence, it's an abstract, a hypothetical question.
You would have to say something like "did you know about it" = visste du om det. (Which cannot be used with the first part of the example as it is worded, they sound unnatural together.)