She's not popular with Dems because she's perceived as fake. She came up as a bit of a tough-on-crime prosecutor, but then when it seemingly was to her advantage in the 2020 primaries, she flipped and tried to get on board with the extremely progressive set. Both sides saw through her, so she has no credibility with either the moderates or the progressives in the party.
She's not popular with the general public because she's a blatant affirmative action hire with very little charisma and a tendency to lose her train of thought and spew absolute gibberish while somehow still sounding like she thinks her listeners are idiots.
She has also been unpopular in the polls throughout her vice presidency, and she performed poorly in the Democratic primaries four years ago.
This election is already over and as a Dems, we only have ourselves to blame for picking such poor candidates.