The main evidence is that Democratic downballot candidates were all generally running well ahead of Biden in the other races in battleground states, in some cases by double-digits:

axios.com - Biden trump swing states polling democrats

This has led some observers to conclude that voters were generally happy to support Democratic candidates, and thus presumably Democratic policies, even while being unhappy with Biden.

He's also had historically bad unfavorability ratings, while other Democratic candidates have had normal ones - which again is a point of evidence that Biden's got some problems specific to him that other candidates don't.

That doesn't mean the Dems would necessarily have a better chance to win the WH if they switch to Harris, though, because some of this might be due to discontent with Biden's policies - and as the Veep, she can't separate from those the way that a downballot candidate can. Ruben Gallego doesn't own Biden's border policy the way that Harris does, for example.