"Adopting your appearance would not be enough. He'd have to mimic your gait, your vocal inflection, your very scent.
I doubt even the most skilled actor with the most sophisticated makeup could pass for an imbecile's distant acquaintance."
I googled those words and it seems to be from the TV series Elementary based on the Sherlock Holmes stories.
This is a bit of a guess but I think he means that it would be very difficult for even a skilled actor to give the impression to people that he was an imbecile's distant acquaintance (by just his actions, without actually saying so). You can't let people know who your acquaintances are by just your behaviour.
I haven't looked enough of the transcript to understand the context in which those words are said.
Your identical twin could pass for you (people may think he is you). Your fraternal twin of the same gender might be able to pass for you. Your brothers/sisters less so. And so on and so on. The speaker carries the idea to an absurd exaggeration.
The actor trying to portray the imbecile would be so far off in his performance that the actor couldn't pass for the imbecile's twin, brother, family member, friend, friend of a friend, acquaintance, distant acquaintance.