It is a blunder because White can win a piece instead. The Black queen is in a molasses of its own pieces, and White has a very good chance of trapping it. Capturing the bishop loses this chance, making the move a mistake.
Here is White's best line of play that quickly plants Black a firm loss.
[FEN "r2n1rk1/pp1bppbp/1n4p1/q1N5/3P3N/PB2B2P/1P3PP1/R2Q1RK1 w - - 0 1"]
1. a4 Qb4 2. a5
By playing a4, White threatens to play Bd2 on the next move, winning the Black queen. Moving the Black knight on b6 to clear room for the queen doesn't help. This is because moving it will lose the bishop on d7, which is attacked by the White knight on c5. As such, Black's best move is Qb4. Now White can play a5 and win a piece. Either the Black knight is taken by the pawn, or it moves, and the bishop is lost.
In other words, playing Nxd7 is a blunder because the chance to win a piece, and likely the game as a result, is lost. This teaches that it is very important to look out for any threats that you can make.