It sure is. The Peace Corps is an agency of the US Federal Government and the whole idea is American citizens representing America in other countries. It wouldn't make sense to represent America if you weren't an American.
From the Peace Corps website:
"The minimum requirements for Peace Corps are:
18 years of age or older
A four-year college degree, or solid work experience in an area such as agriculture, business management, or a skilled trade. A community college degree paired with specific skills or work experience may also qualify you."