How import mysqldump .sql file into MySQL 5.7 when exported from MySQL 8

You are suppose to remove utf8mb4 character set & collation, and add relevant character set in your backup file & then try to restore.

Like :

:%s/SET character_set_client  = utf8mb4/SET character_set_client  = latin1/
:%s/SET character_set_results = utf8mb4/SET character_set_results = latin1/
:%s/SET collation_connection  = utf8mb4_general_ci/SET collation_connection  = latin1_swedish_ci/

:wq 

I think still you will face issue for mysql schema restoration as its architecture is changed in MySQL 8.0 than MySQL 5.7