You can use kubectl get secrets/db-user-pass -o yaml or -o json where you’ll see the base64-encoded username and password. You can then copy the value and decode it with something like echo <ENCODED_VALUE> | base64 -D (Mac OS X).
A more compact one-liner for this:
kubectl get secrets/db-user-pass --template={{.data.password}} | base64 -D
and likewise for the username:
kubectl get secrets/db-user-pass --template={{.data.username}} | base64 -D
Note: on GNU/Linux, the base64 flag is -d, not -D.