OP, I think it's fair to say a lot of people have very little idea of their own effectiveness in a job, whether that's massively overestimating their own capacities, or, as is sometimes the case with impostor syndrome, underestimating them.
People do throw around 'impostor syndrome' as a way of trying to encourage people to think they're good, but in fact there's no real evidence to suggest that people who think they're bad at their job are any more likely to be right or wrong about it than people who (possibly mistakenly) think they're wonderful.
I remember once an excruciating situation with a lovely colleague in a former job (academia), who was shambolically disorganised, and continually had student complaints about his disorganised teaching, sketchy materials, lateness returning work, missed meetings. The departmental headship circulated biannually in his very small department, and when it was about to be his turn again, a colleague in my (allied) department, Y, said jokingly to the others in his department, in front of X 'I bet you're really looking forward to X being Head!'
They tactfully just smiled and turned the conversation, knowing perfectly well what he meant, but X came to our shared office later on and asked Y what he had meant, really huffily, and was apparently terribly upset when Y said 'Oh, well, you're a bit disorganised!'
He apparently had had no idea that he was widely (and rightly) regarded as an administrative disaster, and that he made everyone's lives much harder when he was Head.
This was an intelligent man, and a nice one, but he had no idea he was bad at his job.