If you look at the way the Telegraph, and several other papers can report that Johnson does not want an election, then report that he is calling for one, and then report that Corbyn is a hypocrite for rejecting it, you can see what is meant when people say Corbyn is a bad leader.
The British media is overwhelmingly middle-class and privately educated. They are winners from the UK's economic course over the last 30 years. Anyone who hints at reversing this, or offers a new vision, will be a target - for them all. Liberal papers like the Guardian will even tend to criticise them more than support them. It is nothing to do with Corbyn personally - Ed Miliband, who was not even as radical as Corbyn, was similarly smeared as dangerous, Marxist, as uncharismatic, and as harbouring foreign sympathies. If he had stuck around for longer, the coverage would eventually have moved onto racism and him being a national security risk.
When people say "Labour need a better leader", they're saying "Labour need a centrist leader" - because that's the only way they're not going to be savaged every day across the entire media.
And that's an admission that the scope of political choice and possibility is, and should be, dictated to 70 million people by a small cabal of a few-thousand well-to-do Londoners.