There are several problems making comparisons with Labour Leaders performances in the past.
Firstly there is the respective quality of the other party leaders.Previous Labour leaders were up against not just Tory leaders who were much better at interviews or debates than May or Johnson such as Thatcher Major Hague Howard or Cameron but also Lib Dem leaders the Gang of Four,Ashdown,Kennedy and Clegg who were much better at interviews and debates than Farron and Swinson.
Secondly whether the Tories or Labour were the govt or opposition matters in elections.
Thirdly i would contend that the British education system with each generation since the cold war ended has been run on a more and more left wing agenda and this gives Labour an edge in getting votes of people under 45.
In my view if Andy Burnham had been voted Leader in 2015 Labour would have done much better in the 2017 or 2019 elections.