Somebody (who knew him from his Labour days) said Mandelson was duplicitous, unprincipled, fawning, untrustworthy, cynical and generally slimy. In other words, he was the perfect person to ingratiate himself with a Trump administration and get positive results for the UK.
Personally, I tend to think we have missed a trick here.
PS Clearly even turning a blind eye to Epstein's activities would be reprehensible, but Mandelson is known to be gay and therefore highly unlikely to have been involved in them. Whether Mandelson was aware of them is less certain but I'd assume Epstein wouldn't have advertised them to non-participants.