AWS Cloud Discovery returns Tag Values (Key Values) only for Virtual Machines, Compute Security Groups and Storage volumes.
As of New York release, Tag discovery is only supported for:
Virtual Machines, Compute Security Group and Storage Volume.
It is NOT supported for other resources, for example:
Image, Load Balancers, S3, Cloud Database, etc.
If the feature is important to you, we highly recommend you to follow KB below to create an enhancement request:
Requesting ServiceNow product enhancements