How is this a mistake?


Because it wiped out most of your advantage.
27.. Bc5 would have forced your opponent to move his queen off the diagonal leading to the king. THEN, Nxc2 would have been check.
The way you did it, there was no check. White could have played Qd5+, saving his queen.

I failed to see that move, which is great. But my black knight would still be in the diagonal, and with two rooks on the horizontal, a mate wouldn't have materialized within a couple of moves. I figure trading a pawn/knight for a rook might be advantageous. In any case, my opponent blundered and gave up the queen and then resigned.

@JVG29 That's definitely good reasoning. There was also an issue with my opponent being down to about 30 seconds on their clock, and at our level, I don't think either of us necessarily can think that many moves ahead with one eye on the timer, which worked out well for me. But thanks for the lesson.

Obviously it's a mistake; the best move is to pin the White Queen to the White King!!! Nothing better. Bc4!!
There is no Bishop going to c4. Did you mean Bc5? I would not say, that is a good move, because it does not pin the white queen, the knight is still on e6 and the queen can easyly take on d5 whith check.
The knightmove is not good because just of this move, Qxd5 and then Txe7 and slowly the noose is tightening.