No. Mormonism is a Restorationist Christian religious movement.
Followers of Mormonism come from many different ethnic groups
around the world.
It is true that some religious groups can double as ethnic
groups (such as Judaism - although you can be an ethnic Jew but not
a religious Jew, or religiously Jewish but not ethnically Jewish)
but Mormonism has not had the thousands of years of intermarrying
required to become an ethnic group.
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Costa Rica's main belief systems are Christianity, Buddhism,
Hinduism, Judaism, Bahá'í, Islam, and Mormonism, so roughly
seven.