I think there is an element of hysteria to this story. Let's be clear the criticism of Starmer is in that there was insufficient due diligence on Mandelson before appointing him as the UK Ambassador to the US. To be fair Starmer would not have been doing the DD himself- that would have been given to a specialist team, so that's not directly Starmer's fault.
Also going by the level of news coverage you would think that it was Starmer who was chums with Epstein and hanging out at his island. Whereas in reality Starmer just knew someone who was (Mandelson) and gave him a job.
Sadly it's another example of the level of polarisation in today's politics where facts don't matter, what matters is which political camp you are in.