Yeah, Pancham is called the Naughty Pokemon in Japan, too. The evolution method and it gaining the Dark-type is meant to convey a child growing up with the wrong crowd of people influencing its behaviour and in most cases people that hang around with the wrong crowd of people are usually more inclined towards that sort of behaviour in the first place. So the same scenario applies to Pancham. It's already naughty and mischievous and only gets worse when in a group with a Dark-type. Sort of how two naughty children in a class always seem to play off of each other and act worse than if there is only one naughty child alone with no one to seek rapport from.
It's a basic idea of evolution, to require another type in the party, so I'm surprised it's taken so long to be included. I do like it though, it's more characterful than reaching a certain level.