That depends on where you are. The purpose of certification exams is to test practical interpreting ability to determine whether it meets a certain minimum standard. 1827, the Court Interpreters Act of 1978. So far there are federal certification programs for Spanish, Haitian Creole and Navajo. In the states the situation varies widely.
Many states have no certification at all.
Some are trying. The National Center for State Courts has organized a consortium of states that is sharing resources for establishing standards and testing. Approximately 25 state and local court systems have joined the Consortium. The value of interpreter certification, moreover, depends on the validity of the testing methodology and other certification requirements. Some states use court interpreter examinations that are of dubious validity.