The pollsters Redfield & Wilton have been doing head-to-head polling between Sunak and Starmer since mid 2020, when they started anticipating that Boris Johnson would have to step down. Here are the polls over time:
For a long time, while Sunak was Chancellor of the Exchequer, he was more popular than Starmer. What depressed his popularity was the widespread coverage of his wife's (perfectly legal) non-dom status. Starmer "forgetting" to declare he owned 11 acres in Sussex didn't get similar coverage.
Sunak actually becoming Prime Minister seems to have restored his popularity. He fits the job.
But there is an element of hope there. People who voted Conservative in 2019 really hope he can turn things round, he has goodwill from them, which he should try not to squander.