This is an interesting debate, whether media bias, i.e. the Tory press vilifying Corbyn, is responsible for his low approval ratings.
The problem with this is that the decline in his popularity has been pretty universal, independent of newspaper readership. https://twitter.com/policysketch/status/1198628311108378624

This is BES panel data, with the first wave after Corbyn took over, then the post-2017GE wave when his popularity peaked, and the last one this summer.
The pattern is also the same when we look at changes in perceived competence of Corbyn.

Only here perceived competence increased during the 2017 campaign. But the rise and fall has occurred in the minds of readers of pro-Labour papers (Mirror), pro-Tories papers or pro-Remain papers. And also, notably among non-readers.

Which paper you read determines how much more or less competent or likeably you find him, but not how that assessment has changed over time.