In a recent back and forth with Google’s John Mueller (one of the company’s top SEO researchers), someone asked if having a degree of unhelpful content can pull down the performance of the domain as a whole.
Mueller responded that indeed it can. He qualified this however by saying it would have to be on an important page (or pages), like the home page. If the homepage is “terrible”, Mueller said, then that is a “pretty big deal” when it comes to SEO. He added that “If the same site has a random archived page that’s ‘terrible,’ no big deal.”
Bottom line: It’s not so much about “terrible” as it is about unhelpful. Keep your site’s content of a high quality, sure, but even more importantly perhaps is to ensure it is relevant to your offering or what you are promoting. It’s pretty obvious if you think about it: If your site gets “watered down” by poor and irrelevant content, visitor behaviour is going to reflect that, and Google’s algorithm will in turn punish you when it comes to SERP rankings.