How/[is it possible] to get the results of smart campaigns using the API?
I'm interested in the emails sends, opens, unsubscribes, clicks, etc.
I don't think there's a way to filter which smart campaign you want results for in the API request. I would try querying those activity types (sends, opens, etc.) and then see if the results return a value for the ID or name of a smart campaign that's responsible for the change: http://developers.marketo.com/rest-api/lead-database/activities/#query
Apologies if that information isn't part of the response - I haven't tried it myself. Maybe also try the bulk activity API to see if it returns different data. http://developers.marketo.com/rest-api/bulk-extract/bulk-activity-extract/
Like Grant says, a Smart Campaign's Results tab does not have a (directly) corresponding API endpoint.
You must filter the Activity Log by Smart Campaign to simulate the output. Don't expect full fidelity as SC name changes aren't captured.