- are all of your records synced to SFDC?
- Smart List should be SFDC Type=Contact, Has Opp=F
If you specifically want to track people who were converted in a time frame,then your smart list should provide some detail. Marketo may not understand that you want Contacts w/o opps. All it sees in that list are people it knows were converted, which it may not know properly before you connected the systems.