Mueller's mandate is to investigate Trump, not Clinton.  That has nothing to do with being "impartial" - only with doing the job that's been given to him.

And, much as I dislike Hillary, fact is, the GOP have been trying to find dirt on her for over twenty years with no luck - even after spending hundreds of millions of dollars of taxpayer money in the process all those Republican led committees came up with zilch.  That means that either (i) there is nothing to find or (ii) the Republicans are completely incompetent.  Or maybe it's both?