Mack doesn't have to have met Lucy to know about her. Karen was his wife, and Gary and Val were his friends and neighbours. When Lucy did appear in Knots Landing in that Season 1 episode, she was well outside her comfort zone and it really showed. That worked well in terms of the storyline, showing how Lucy's upbringing on Southfork and the spoiled rich girl mentality that she had, had made her very different from the different outlooks of both of her parents. Gary and Val have differences in their outlooks and experiences, but they are both used to types of hardship. Lucy would have fitted into Knots Landing a lot better in the later seasons, but Seasons 1-3 of Knots Landing (before Jock's Will and Gary coming into money), did not suit Lucy at all. I think that's just a rumour. Val gave birth to Lucy when she was 15 or 16, so Val isn't supposed to look much older than Lucy anyway. Karen even made a joke that they looked like sisters.