I'm not sure where the blame lies. Ultimately it's Thompson's fault for poor drafting of OL. Thompson seems to communicate well with his coaches and scouts though, relying on them to say yea or nay on a particular player. In the Nate Palmer thread, there's an article where Palmer said he didn't think he would be drafted, but Kevin Greene seemed taken with him, McCarthy and Capers took Greene's assessment, passed on the info, and they picked him. My guess is that is the norm with this team and many teams.
I'm guessing McCarthy and/or Campen say they want a player with a certain skill set for a certain position. The scouts find those players and the Thompson and the coaches continue the research, as Kevin Greene did with Palmer. They say yea or nay and Thompson drafts accordingly. I doubt Campen gets stuck with players he didn't want or hadn't been exposed to.