He's polling worse than Corbyn. You were wrong.
Starmer's last net satisfaction score was -29 which ain't great, but Corbyn hit -60 at one point which is the lowest of any party leader in the last 45 years.
Corbyn was probably the hardest hit victim of this pathetic and miserable right wing trope of "if you don't support Israel's illegal occupation and express any sympathy with displaced Palestinians you're an anti-semite", and the UK voting populace is in large part motivated by small minded pettiness, greed, selfishness and delusion, so I'm not saying for a moment that Corbyn was a bad guy or that I don't have sympathy with a number of his positions. But he was completely unelectable. He comes from a long line of unelectable Labour leaders such as Foot and Kinnock; the trade unions can't carry an election any more and appealing to the left wing loses you the massive middle.
There's a reason that Blair was the only successful Labour leader in my lifetime. Wishing the UK had a greater sympathy with socialist causes will not make it so.