Topping, or hitting the top of your ball with the bottom of your club, is much more common for newer golfers meaning we generally grow out of it. However- the slightly less ugly shot are just thin shots which we can always do and they are basically topping but just a little better. These shots are generally caused by swing too far outside to inside. For a right handed golfer- your shoulders are pointing too far to the left at impact and your swing is going too far to the left which causes you to hit the ground too far after the ball. You can also top it if you have early extension, or if your hips move in toward the ball on the down swing causing you to stand up and loose your posture. Your topping may also be caused by just swing too hard with just your arms and you unhinge way to soon causing you to swing way too early in your swing.
Drills: Hit balls with your feet together so you can’t move around too much and you learn to find the bottom of your swing.
Find some “Sand”, draw a line in the sand slightly forward of the middle of your stance and perpendicular to your feet. Take a 9 iron and start making full swings taking all the sand from the line forward. You have to hit sand but you can’t hit behind the line. Best golf drill there is in my opinion.