Well, the fault does lie in the writer in that they were such a crappy writer I ignored there shitty teen characters to the point where I couldn't be bothered to remember the name of their group. If they weren't some of the most aggressively annoying characters this season, maybe they would have been worth paying attention to. Instead, to me their just annoying Scottish villagers in one of the most bland, average Doctor Who episodes in awhile. I'll only remember them because them being terribly written YA film style characters is one of the few things memorable (in a bad way) about this episode. So, yeah, definitely the writers fault.
Really? Everyone? They're not exactly Romans or vikings. Even in the UK I would bet thousands of people (if not more) have never heard of them, much less in the US. This comment was ridiculous enough that I did glance at their wikipedia page, and even it calls them very similar to most of the other cultures of the area at the time they existed, and they just kind of faded away. So, I'm calling BS on even most people in the UK knowing about the Picts, much less expecting them to be common knowledge to people in my country (and no, you can't prove they're common knowledge just because some people on a Star Trek forum know about them).