Blunder for White in a Queen's Gambit Declined?

Concerning the movement of the knight, no, I would say that this is neither a blunder nor a "terrible mistake". Usually, when you commit a blunder, it is immediately obvious that you have done so. This game is in a slower, positional phase... and so any "mistake" that you make now is likely to be a small mistake.
Actually, I would say the knight is well placed. It gives you another defensive option at c3, and it avoids two minor threats to you pawn structure. 10. Nf3 Bg4 or 10. Nf3 g5 11. Bg3 Ne4 (or Nh5)
The Castling question is perhaps more important, and I would rely on a more learned opinion than my own. I think 0-0-0 is better, or at least more interesting. ATM it is a very safe castle, whereas black's structure suggests that his peices have better access to the kingside than your pieces do (danger). 0-0-0 might also create better attacking possiblities for white. After all, the "luft" in black's castle suggests attacking here. 0-0-0 would leave your king out of the kingside complications, and would allow you to more easily bring a second rook to the kingside.