for me issue got resolved, by adding MapJsonbAsClob=true; in the source endpoint.
what it does consider the jsonb as NCLOB.
refer to below doc:-
https://docs.aws.amazon.com/dms/latest/APIReference/API_PostgreSQLSettings.html
https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Source.PostgreSQL.html#CHAP_Source.PostgreSQL.Prerequisites