Hi Justin,
There is no concept of a default lead source value for importing of leads in Marketo.
For best practice, when you import a list, add a column to the spreadsheet for Lead Source and whatever value you want it to be for that list you are importing.
SFDC does have the concept of default values for fields. Because you are not setting a lead source value in Marketo, when the lead is first pushed to SFDC and created there, since the lead source field is empty, SFDC will apply the default value.
Regards,
Sheila