We as humans often assess another's intelligence by how they speak, and with politicians, that's about all we have. I think it's reasonably valid. I wouldn't call Harris "dumb" exactly, but she does sound pretty dumb at times. She did fail the bar exam the first time, which is, well, a something. She may well have average intelligence, or near that, and at times struggle to communicate tersely and without sounding a bit lost.
All that said, I suspect if she's elected, her decisions will be made for her. She'll be a prop, a face, to appear to be President.