what are you trying to import? Image URLs?
ACF image fields hold image (attachment) ID values.
The postmeta name will be the same as the field name unless the field is a sub field of some other field like a repeater. So importing correctly depends on getting the field name correct.
There is also the ACF field key information that needs to be added to the database so that ACF knows what to do with the field value, are you adding this information to the database?
