In large part this is due to Republican's framing the argument.

"she didn't go to the border-BAD"

then "she went to the border-BAD"

Give me a break.

There's a large touch of mysoginy at play as well . Typical double standard when it comes to women in positions of authority or power.

Quote: The decline also corresponds with studies showing that Harris, like other female politicians, has increasingly become the target of online abuse. As Harris’ stature has increased, so has the volume of sexist, violent and misogynistic attacks against her on social media, with researchers finding hundreds of thousands of examples [1] As someone pointed out in other comments, VPs don't usually see great poll numbers anyway and Harris' pretty much match (so far) the trajectory of the previous four VPs

[1]

https://www.latimes.com/projects/kamala-harris-approval-rating-polls-vs-biden-other-vps/