The collapse has prompted Mr Corbyn to stand down, as Labour activists come to terms with the catastrophic results.
Ms Parker apologised on the BBC to Momentum activists following the "deeply disappointing result" before she urged a period of reflection for the Labour Party.
Maitilis hit back at this: “Normally a period of reflection would be understandable. But you were warned so many times, so often, in such clear language from your own MPs from people who defected, from people on the doorstep, from people who tried to sell the Labour message.
“What part of that could you not hear?”
Maitlis cited former Labour home secretary Alan Johnson's earlier attack on Jeremy Corbyn’s leadership as a “disaster”.
Mr Johnson had said that Mr Corbyn "couldn’t lead the working class out of a paper bag".
Maitlis added: "If your Labour leader can’t do that, what is Labour for?
"You lost two and a half million voters in two years. How do you do that?"
Ms Parker tried to defend Mr Corbyn saying: "This was the leader that took us to the largest increase since 1945 in 2017.
"There is no denying we have had a very difficult two years. Brexit has sucked the life out of politics and we have not managed to cut through the Brexit noise."
Maitilis also grilled Ms Parker on claims that Momentum had became "a cult".
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Ms Parker denied this description, prompting Maitlis to call it "a bubble".
The Momentum leader said that she believed Labour would be able to regain the votes in the Red Wall that Labour had lost to the Conservatives.
During his acceptance speech, Mr Corbyn said he will remain in place as Labour leader while his party undertakes a “period of reflection”.
Mr Corbyn pinned the blame for Labour's poor showing on Brexit before criticising media "attacks" towards himself, his family and the party.