As individual items they polled well.
In 2017 he took away the Conservative party's majority in the House of Commons.
The real two reasons why Labour took a pounding is Brexit and Corbyn himself.
1) In 2016 Corbyn did not do a good job promoting the need to remain in the EU as he was an old school socialist who never wanted to join the EU in the first place. But 70% of his party voted to remain. In 2017 Corbyn said he'd respect the result of Brexit but didn't elaborate what that meant. Labour leavers in key northern seats stuck by him because they felt he was on their side. Labour remainers were mostly in London and University towns which are safe Labour seats anyway. But in 2019 he said he'd support a second vote. Labour leavers felt betrayed and voted for either the Tories for the first time or the Brexit Party (Nigel Farage). Farage's new party won no seats but they were a damaging protest vote.
2) Corbyn was seen as a weak leader. He got attacked viciously by the press and never fought back the charges. He was seen as weak on national security because he in the past said Britain's nuclear weapon defense program was a waste of money and morally wrong. He was seen as a traitor to patrioticism because he met with the IRA, Hamas and Hezbollah while criticising the British army for war crimes. He couldn't stop antisemitism growing under his leadership. He already had to fend off a vote of no confidence from his own MPs. He was basically made out to be anti-British, open borders, weak on crime, economically damaging communist and that campaign evidently worked.
But I repeat, the policies as individual items polled well. Maybe a new, less problematic leader can sell them better. A new Labour leader will still get destroyed by the right wing press. Ed Miliband had it just as bad, if not worse, as they targetted Miliband's deceased father who was a Jewish British WW2 veteran who fled persecution in Nazi occupied Belgium.
The key is also not to overpromise. Too much too soon is bad. Tony Blair won in 1997 by being charismatic, young and media saavy. Blair won in 1997 with his "education, education, education" mantra. Winning an election is not about out-promising your opponent because you're just going to disappoint everyone and fail once in government. And you might not even get in government as people stop taking you seriously. Focus on a few key policies, hammer them every day, have a good media response team. Do the work and the voters will follow.