This question can be argued on both sides.
It is true in the sense that you have a motivation to study hard when you have set up a goal for yourself. Unless you already have achieved those goals, you eill have a motivation to study hard.
This can be majorly false. Speaking from personal experiences, I only study hard when I have a A in a class and I want to get it up to an A+ Once I have all A+ (or mostly) I try to maintain it with minimal efforts (I don't recommend this). I think setting your goals that low can never get you motivated towards studying. The desire to get higher grades may only affect your daily homework doing skills, but I haven't noticed myself studying more just by setting a grade goal.
You can increase your studying by a greater motivation of making changes to the world like great minds did.