Hillary had a weird double-whammy of underestimating the appeal of Trump for many that led to losing control of the monster she helped make, along with having a long list of insults ready for anyone who didn't want her to be the Democratic candidate that didn't endear her to the voters who could have made her presidency for her. Whether it was calling them deplorables, broadly dismissing any criticism of her within the party as rooted in misogyny, or accusing them of being unrealistic idealists with pie in the sky goals and unelectable candidates, she really had a knack for taking these people and firmly putting them in the camp of "Screw her, I'm not voting for someone who treats me like that." rather than engaging in a serious attempt to understand these voters and address their concerns.

Democrats today have certainly learned that Trump could be a serious threat, not to be dismissed out of hand. To his credit, Biden has notably not fallen into the sort of self-destructive antagonism of the electorate that is not already firmly committed. He might pay only lip service to their concerns, but I'm not aware of him blanket writing off, say, pro-Palestinian protestors en masse as antisemites that were never going to vote for him and are beyond redeem, even if he does frequently trot out manufactured claims of widespread antisemitism.

People online trying to drum up support for him don't seem to have gotten the message that this didn't work out so well for Hillary, and are going at it, calling people who haven't vocally committed to Biden anything from idiots to Russian shills to Republican trolls, and claiming they hate minorities and LGBTQ+ people or whatever else occurs to them to rile up people. I don't see that working out to their advantage, and predict it will alienate people who might have potentially been won over.