Are mistakes a sign of improvement?

I recently flipped through Andrew Soltis' book The wisest things ever said about chess. One of the quotes, attributed to GM Loek van Wely, says the following:

"You only know that you are improving when your opponents seem to be playing badly more often than before"

Earlier this week, I played a game against an opponent that I have struggled against before. This time, it went better. Not only did I manage to win quite easily, I also noticed that my opponent played some rather peculiar moves, and I felt quite confident throughout the game. This was a quite new experience for me, and hopefully, it is a sign that my understanding of the game has improved.

The game began as a Dutch defense. This is an opening I have never played as black, and I have only played against it occasionally. So my knowledge of theory was basically zero.

I did not really know what I was doing, so I tried to create a fairly familiar pawn structure, and develop my pieces to active squares. Seven moves into the game, my opponent plays 7... Nc6, allowing me to push d5 with tempo. At first, I thought that there was some kind of tactic against d5, but it did not seem to work, so I simply pushed the pawn.

As soon as the knight went back to b8, I felt that I should have a clear advantage. Sure, my pawn structure looks silly. All my dark squares are potentially weak, but this is only an issue if my dark squared bishop is traded off. And if we trade DSBs, my opponent will have weak dark squares around the king, which seems like a bigger problem. Further, my pawns take up a lot of space, and control/occupy the center of the board. I felt that things should be going my way.

Things were going fine until my opponent started to untangle and got his knight out. After 14... Na6, I should have played Bxa6. But with Silman's words "You have it, he doesn't" ringing in my head, I wanted to preserve the bishop and possibly use it to generate a dangerous attack. After my not so strong move 15. O-O, the computer tells me that whatever advantage I had is now gone since the knight as a beautiful outpost on b4 (in addition to the dead d-pawn).

A few moves later, it is time for my next mistake. I thought I would cramp up black's position by planting my rook on d6. And in some variations, that may have been the case, but this was not one of those variations.