-
It's a bad rewrite of this: http://www.independent.co.uk/news/uk/politics/poll-second-eu-referendum-brexit-theresa-may-general-election-voters-a7140721.html
Politicshome was owned by Lord Ashcroft until 2011, I expect the editorial to retain at least a little bias to the right-wing.
-
It looks like the underlying poll was commissioned by the Independent/Mirror so no particular right-wing bias there. http://www.comres.co.uk/wp-content/uploads/2016/07/Independent_SundayMirror_July2016-Poll_tables.pdf Even those who voted for Labour in the 2015 election have May as the better prime minister (40% to 36%).
He's also viewed as less principled than May which can't be a good thing
Do the party members really matter though? It's the general public that need to be convinced if he is to achieve anything.
This https://www.politicshome.com/news/uk/politics/opinion-polls/news/77380/poll-theresa-may-has-big-lead-over-jeremy-corbyn (a bit of a poor article in terms of details of the surveys but the first thing that came up with a quick google) suggest that he is behind May but also "Mr Corbyn also narrowly trailed leadership rivals Owen Smith and Angela Eagle on who was more likely to win a general election."