This is one of the standard fields that we do not sync from SFDC. As this is a lookup field you would only see the 18 Character cross reference ID in marketo (i.e. the Account record ID of the Parent Account in SFDC starting with 001)
If you would like to have the ParentAccount Name displayed in Marketo then you can create a custom field in SFDC to hold the Parent Account Name. You can have the new field as a formula or use a Workflow or Apex Trigger to update this field going forward to ensure it is correct.
To get this information into Marketo you need to make sure that any of the Account Records that have the partent account name get updated as we only sync records over that have been updated. You can use dataloader or the like to do this. You SFDC Admin can help you with this. You only need to make sure the Timestamp is updated you dont actually need to change any information on the record so doing an update with just the record ID's usually is sufficient.