The most common reason that I see for topped shots is most beginners and higher handicap players are trying to lift the ball up in the air with an ascending swing.
When the ball sits on the ground it requires a descending swing, when the ball sits 2 or 3 inches up in the air for a Driver it requires an ascending swing.
I work on impact a lot with my students and I stress understanding that the low point of the swing needs to be on the target side of the ball. We want ball then turf contact. I have them make a lot of practice swings during the lesson seeing and feeling the club bottom out on the target side of the ball and then ask them to work on this at home with a mat or even an old piece of carpet.
The PGA Tour average angle of attack with an 8 iron is 4.5 degrees down. The LPGA Tour average angle of attack with an 8 iron is 3.1 degrees down. When I get beginners and higher handicap players on my Launch Monitor their angle of attack can be anywhere from 2 degrees to even 6 degrees up. They tend to have the classic hang back and lift golf swing.