But I'm guessing that you've hit on the right direction of interpretation of sloppiness as letting go of control. I'd extend that to too much self conscious* control. Great art, and particularly great dancing, is finding a clear intention and a method of focusing your discursive consciousness and voluntary attention that harnesses the rest of your capabilities for the same intention.

That seems related with the common observation that it's easier to speak a foreign language when drunk than when sober: in the latter case I feel I'm so worried of saying something grammatically incorrect that I end up speaking in very simple sentences and very haltingly. (And the widespread use of drugs among rock musicians is well-known.)