I think you're right about a lot of the "Kamala might win" attitude being very vibes-based and based wholly on DNC propaganda and astroturfing, but I also think the situation with Hillary was very different. Hillary had been hated for years, and in 2016 that all caught up to her. A big part of Bernie's platform was opposition to neoliberalism, for which Hillary was a posterchild. When Trump went against her in the election, he lifted a bunch of Bernie's talking points and was able to attack Hillary simultaneously from the right and left, and he really seized on her unlikeability.

Kamala doesn't really have the same problem since she's less nationally known and a lot of people don't really have a preformed opinion on her. Even as Biden's VP it seems she's mostly flown under the radar and the Dems have been able to present her as some sort of tabula rasa. Also, thus far Trump's angle has been very weird, the "childless cat ladies" thing, the "I didn't know she was Black" thing, all a far cry from Trump's antiestablishment attacks on Hillary which were popular on both sides of the aisle. The best thing I've seen from him is his point that she was chosen undemocratically to take over from Biden, but unfortunately Biden was one of the least popular presidents in US history and most Dems are glad he's not running. It's a very small amount of people who were planning on voting for Biden but now won't vote for Kamala based on principle.