No, they can't be used in content filtering.
Network Objects/Groups can be applied in the below firewall rules:
Individual and Template Networks: Layer 3 Inbound, Layer 3 Outbound, and Failover Cellular Firewall Rules
Organization-wide Site to Site VPN Outbound Firewall Rules
https://documentation.meraki.com/MX/Firewall_and_Traffic_Shaping/Network_Objects_Configuration_Guide