In the United States, does the Clerk of Court typically have any type of decision-setting power, or does the Clerk's authority extend only to carrying out directives of judges?
The answer is going to vary from state to state and, even within a state, from jurisdiction to jurisdiction.
However, in general terms, non-judicial court officers, such as clerks of court and prothonotaries, will have specific tasks delegated to them by the court; they can make decisions on matters that the legal system does not consider to require judicial discretion and judgment. These are often described as "ministerial." They may include significant decisions with significant consequences: for example, dismissing a case, or granting judgment against a party, where a party has missed a deadline. In some if not all cases, these decisions can be appealed to a judge, but are unlikely to be overruled.
There are also another class of non-judges, such as magistrate judges and administrative law judges, who take on more traditional, non-ministerial judicial roles, acting and making decisions that call for the exercise of judgment; again, these are generally appealable to a regularly appointed or elected judge.
The short answer, then, is: Clerks of court cannot do everything a judge can do, but they can act on their own discretion when the matter falls within their ministerial authority. This authority can include, in appropriate cases, deciding who wins and who loses a lawsuit.