When light cruisers made sense as destroyer leaders

Were dedicated light cruisers used as destroyer flotilla leaders a flawed concept?

No. They were viable in the First World War, but large destroyers could perform the role more efficiently by the Second.

Purpose-built light cruisers leading destroyer flotillas were not an inherently bad idea. Their command space and firepower suited First World War conditions. By the Second World War, however, large destroyers such as the Tribal class could perform much of the same work. The role persisted while the most efficient hull for carrying it changed.