I'd expect to be able to do something like:

targetCustomizations:
  - name: region-a
    clusterSelector:
      matchLabels:
        region: a
      helm:
        values:
          regionSpecificKey: region-a-value
  - name: region-b
    clusterSelector:
      matchLabels:
        region: b
      helm:
        values:
          regionSpecificKey: region-b-value
  - name: dev
    clusterSelector:
      matchLabels:
        environment: dev
      helm:
        values:
          environmentSpecificKey: dev-value
  - name: prod
    clusterSelector:
      matchLabels:
        environment: prod
      helm:
        values:
          environmentSpecificKey: prod-value

Or at least be able to do something like:

targetCustomizations:
  - name: region-a-dev
    clusterSelector:
      matchLabels:
        region: a
        environment: dev
      helm:
        values:
          regionSpecificKey: region-a-value
          environmentSpecificKey: dev-value
  - name: region-b-dev
    clusterSelector:
      matchLabels:
        region: b
        environment: dev
      helm:
        values:
          regionSpecificKey: region-b-value
          environmentSpecificKey: dev-value
  - name: region-a-prod
    clusterSelector:
      matchLabels:
        region: a
        environment: prod
      helm:
        values:
          regionSpecificKey: region-a-value
          environmentSpecificKey: prod-value
  - name: region-b-prod
    clusterSelector:
      matchLabels:
        region: b
        environment: prod
      helm:
        values:
          regionSpecificKey: region-b-value
          environmentSpecificKey: prod-value

But neither of these seem to be an option because it's first match wins (no composition), and OR (not AND) when using multiple labels in a match.