No, you don't have to be certified to be a preparer but you are required to have a PTIN to prepare returns as a paid preparer.

To represent clients, you generally need to be a credential preparer.  Non-credentialed prepares have limited representation rights for clients whose returns they prepared.

See this IRS article for more details: https://www.irs.gov/tax-professionals/understanding-tax-return-preparer-credentials-and-qualificatio...