Yes, that is very important. 4-strokes are a little more forgiving than their 2-stroke counterpart, but anytime power is gained through the motor the clutch needs adjusting to put the power to the ground.
An example that will make it easy to understand: Take a stock scrambler 2-stroke 400. The rpms run at around 5700-5800. If you go and put a trail pipe on it the rpms will need to be run at 6700-6800 to make the power it should. If you dont change the clutching to match then the motor will not turn full rpms on top and wont be much better than stock. There could be times when it wouldn't be any better or actually worse than stock. This example may be a little extreme, but you see the picture.