I believe Six Sigma can be an effective management improvement strategy. It is not the most effective, in my opinion, but still effective. In my opinion, if Six Sigma isn’t making a significant impact on fixing bad management then it isn’t a very effective six sigma effort.
Like most management concepts how it is applied varies tremendously. If one just uses some tools that are part of the “Six Sigma tool kit” (mostly tools from TQM and the like) then you might improve bad management only marginally.
But if you read the work of Roger Hoerl, Soren Bisgaard, Forrest Breyfogle III… and learn and apply what they talk about as Six Sigma you will definitely have to address bad management practices. Their Six Sigma is definitely a management improvement system (you can’t apply their concepts of Six Sigma without fixing many bad management practices).