The Default Contact and Account Source Property in HubSpot is not a form field and is not editable. Create a custom Property in HubSpot to circumvent that limitation, and be able to report by Account Source.
How to Sync Account Source as Apollo in HubSpot
1. Create a Custom Account and/or Contact Source Property in HubSpot
Follow these directives on How to Create Custom Properties in HubSpot
When creating the Contact Source or Account Source Custom Property in HubSpot, be sure to choose the 'Single-line text Field Type.' Other field types cannot be mapped at this time.
2. Map Account and/or Contact Source
Navigate to Settings > Integrations > HubSpot > And, under the Push Settings section, scroll to Contact Source and Account Source. Choose your Property from the dropdown menu, and Customize your Source, most likely as "Apollo" as a the record source.
Related Resources for Setup Best Practices in Apollo: