Throughout the SNP leadership election, polls consistently showed that Kate Forbes was far more popular with the general public than Humza Yousaf. Weirdly, though, Yousaf made several attempts to get those poor polling results to work in his favour by zeroing in on his showing among SNP voters, which he claimed was all that mattered. The claim that he was ahead of Forbes among SNP voters was always based on smoke and mirrors, because only roughly half of polls actually showed that. The other half showed Forbes ahead among SNP voters, which probably means the two leading candidates were roughly level-pegging with each other on that measure. Combined with Forbes' far, far greater popularity among the wider public, that should have been more than enough to seal the deal for her in any party that cared about winning elections and was thinking rationally.
But if Yousaf's claims of popularity among the SNP's own supporters were questionable during the leadership campaign, they're even more questionable now, because the new YouGov poll shows his net approval rating dipping to just +4 among people who voted SNP at the last general election. The best way of putting that into perspective is to compare it to the opposition leaders' net ratings among their own parties' voters from 2019.