The topped golf shot is one that so many golfer hit. The ball shoots or bounce along the ground with no launch or flight. I then hear so many golfers say "head up" after hitting the top shot. This always makes me feel sad because that golfer is never going to improve unless they deal with the issue.
99% of topped golf shots with an iron from the ground are because the player strikes the ball on the way up. The club head has reached it's lowest point before the ball and is on the rise as it strikes, thus encoraging the top. Iron shots from the ground need to be struck on the downswing. If you watch the golf on the TV one of the most asked question is " how do they get so much backspin". To generate backspin a ball has to be struck with a decending blow. It is the downward strike that imparts the backspin on the ball. Worldclass golfer hit the ball hard and on the downswing with iron shots into greens and this gives them control.
To practise hittiing the ball on the downswing and to check if you do hit the ball on the way down or the way up, simply hit some shots with ball 6 inches forward in your stace. Hit some iron shots with the ball forward of your left foot. If you are striking on the downswing your club would still be very low to the ground at this point so hitting a shot with the ball so far forward won't feel to hard. If you find it hard to strike the ball when it is so far forward then you are hitting on the upswing. Try practising this drill until it feels more natural with the ball forward then move it back to the correct poisiton.