I didn't care for her, but Clinton I would say had more than basic competence, though she didn't really have much charisma, is pretty unlikable, forced out Bernie Sanders, chose a completely forgettable running mate, was running as an establishment candidate in a "change" election, and has a blatant desire for power. It all worked against her, though she did still win the popular vote. Still fairly competent though.
I would argue Harris is in a similar situation as Clinton, though she apparently had the sense to at least pick someone more likable than herself and her almost invisible tenure as VP actually may work to her advantage. Otherwise, she's not incredibly charismatic on her own and could be in trouble at the debate with Trump, though his elderly, stroke-addled brain may help her out.
But if it gets into the same sort of thing like in his debate with Biden, where Trump is just throwing out BS bombs left and right, and all Kamala does is just uselessly flail around trying to address his BS points one by one, I think she'll be in trouble. She needs to cut through the BS and present herself as the adult in the room, and make Trump seem infantile and ridiculous.