Favorability ratings aren't relative to each other, they're standalone ratings. They're not unchangeable, but they do tend to be kind of sticky. For a contrast and a picture of the kind of popularity you'd want to see from a candidate, Obama was a +24 net favorable with independents back in October 2007. That's 41 points better than Biden or Sanders are currently faring. People just kind of liked Obama (including me). The same is true across demographic and ideological splits - hell, he was only down 11 points with both self identified Republicans and ideological conservatives. Think about that - right now, every major Democrat candidate is more disliked by independents (and all Americans) than Obama was by Republicans in October 2007.
Basically, all the Democrats needed to do in 2016 was not pick the most disliked candidate in American political history (or at least since we have polling). They failed to do so and somehow they're going to blow that record out of the water. Maybe they'll win anyway since people don't like Trump, but this is a bad plan.