My mother was born in Rijeka, currently in Croatia, and held citizenship while under Yugoslavia. She then emigrated to the US before Croatia gained its independence. Do the laws regarding citizenship by direct descent apply or is there an exception in this case? I am under 21 years old.
Your first task is probably to determine whether your mother is a Croatian citizen. As I understand it, Yugoslavia had a concept of concurrent citizenship (like the US and Switzerland, and probably some other federal countries), so Croatian citizenship existed before independence and persists after. If your mother is a Croatian citizen, or, more precisely, if your mother was a Croatian citizen when she gave birth to you, then you may well have been a Croatian citizen all your life without knowing it. More likely, you may be eligible to register as a Croatian citizen because you are under 21.
If your mother naturalized in the US before you were born, she may have lost her Yugoslav and/or Croatian nationality at that point. Some countries have provisions to that effect in their nationality law; I do not know about Croatia. (Whether such a provision exists now is not relevant; what matters is whether it existed when your mother naturalized. Similarly, whether she could pass Croatian citizenship to you depends on the law as it existed when you were born.)
If you are not already a Croatian citizen and you are unable to register, then you may be able to naturalize. As the descendant of an emigrant, you are exempt from the most burdensome requirements: to renounce your other citizenships, to have lived in Croatia for eight years, and to be proficient in the Croatian language and Latin script.
More detailed information is available on the relevant page of the ministry of the interior. An application from outside of Croatia would be submitted through a consular office of the ministry of foreign affairs; see their website for more information.