I don't think the difference between biden in 2020 and hillary in 2016 was much more other than perceived likability, plus the year they were running in.
4 years ago Biden's favorables were much higher. He hadn't done anything in office that made people mad yet, and with trumps handling of covid i think the 'elder statesman who won't be using his knees to handle the wheel' was a genuine selling point. hillary's campaign was bad, in part because of the lack of seriousness they took it. She is also both unlikeable as a human being, and one of the main villains of the Fox News Cinematic Universe.
In 2024 I think Biden is probably comparable to Hillary (his unfavorability at this moment might be even worse than hillary ever was tbh). a lot of the dings on biden are kinda bs, but with all of those things (economy, israel/palestine, immigration, whatever you want to throw in there) he's ancient as fuck and has struggled largely with messaging. and his admin does not appear to be taking that seriously at all. Every account I hear about the biden admin makes it seem like a room full of yes man who plunge their heads into the sand anytime they hear any feedback or criticism of their decisions, regardless of how good of faith those are made. I think we were witnessing the democratic nominee making the same exact mistakes they made in 2016.
on the other hand, trump is also a significantly weaker candidate than he was 8 years ago. Whatever outsider status he had back then with him completely remaking the republican party in his image. In 2016 he was calling the other candidates morons for supporting the iraq war (not that trump is actually in any way antiwar, ofc) whereas now everything he says and does is right wing grievance politics. He's also a convicted felon, and his campaign message is "me losing the 2020 election is the most unfair thing that ever happened to anyone". I think it's certainly possible trumps polling gets worse as the election gets closer and voters who haven't been really paying attention notice trump and think "ah man this fucking guy again.
Either way both candidates here are really weak. And its a pretty damning indictment of the biden administration, that they are locked in a coughing baby fight with the same guy they intended to defeat once and for all. Pretty much every policy, every component of his style of his politics that trump stands for, will be here for the foreseeable future.