Altium Designer - Should I place ground vias on a ground polygon?
The answer to your question is "No, you do not need to have a via for every pad if they're already connected to the GND pour". 

However having generous amounts of stitching GND vias between layers is a Good Thing. Consider looking at your PCB one layer at a time where it's easier to see if you've got semi-isolated GND islands. Rule check is happy with a single via connecting various GND planes together but your circuitry won't be. 

The screenshot makes me think the blue area is on bottom side and red on top. Switch to a single layer view to see how the top side is connected.