I personally agree with your reservations about Corbyn and his front bench, but I've got to ask...how much is Corbyn's unpopularity with the public due to Corbyn himself, and how much of it is due to the fact a large part of the Labour Party (including most of his parliamentary colleagues) are not very supportive of him? The thing is, the public like "strong leaders" and "united parties". Maybe if Labour was more united around Corbyn, he'd have a better chance with the electorate?
Probably, but still he's seen as weak and not Prime Minister material because he, among other things: refuses to entertain the idea of pressing the button; refuses to accept responsibility for protecting the Falklands; refuses to stake out that the UK should stay together; deflects to 'negotiation' whenever asked a foreign policy question (you can't negotiate with ISIS); chose a buffoon for Shadow Home Secretary; has his stooges paint the conservatives as some kind of sadistic reptilian monsters; scaremongers with the NHS then preaches about scaremongering with immigration; dresses like a homeless person; wants to damage our environment by reopening coal mines which would go bust almost immediately, while acting disgusted at people who going hunting; wants rent controls, which have never worked; takes Tory manifesto pledges like the triple lock and masquerades them as his own; and then he has a shadow chancellor who attends communist rallies, idolizes Marx, Trotsky and Lenin and called on employment minister Esther McVey to be lynched but then never didn't apologized. He'd just not a good leader.
What has Corbyn's personality got to do with the manifesto? I think you've basically agreed with me- the manifesto is generally popular.
The point of a manifesto is to make a party in an election more appealing. But the manifesto hasn't solved any of their main problems.