Non era così male.

English Translation

You weren’t that bad.

Isn’t male an adverb?

Well, yes. But the Italian sentence is natural, and that’s how we express this in Italian.

You cannot use “cattivo” in this context.

However the sentence has a primary meaning of “It wasn’t so bad”, it may mean “you weren’t so bad” if you think it is using a formal “you”, or anyway, it is an encouraging sentence you can use when someone thinks he gave a poor performance.