I was reading the Wikipedia entry on the use of
False Flag during war, and it seems it was considered acceptable practice by both sides that as long as the false flag was lowered and the national flag raised before engaging in combat.
The Kormoran cut things rather fine, but no more so than the HMS Campbeltown did at St Nazaire a few months later.
If the situation regarding the Kormoran and the Sydney was reversed and an Australian Q-Ship had suckered an unwary German light cruiser, I'm sure the Aussies would be treated as national heroes.