Why is City Hall not a path to the top? Is it looked down on? Why should Congress be a more reliable inside path? Surely a mayor of say NYC could offer valid qualifications?
I'm not necessarily referring to Guoliani.
Many Presidents come from small towns. Big city politics was never an option for them, and small town politics wouldn't ever have interested them enough, and they had sufficient ambition and talent to skip them for other qualifying paths. See: Bill Clinton, Lyndon Johnson and Richard Nixon (first campaigns were for Congress), Jimmy Carter (first campaign was for State Senate), Woodrow Wilson (first campaign for Governor).
Even big city people ambitious and connected enough to be President can, and do, skip the local government stage of their political careers. Barack Obama (Chicago), George W. Bush (Dallas), George H.W. Bush (Houston), Ronald Reagan (Los Angeles), Franklin Delano Roosevelt (New York), Hebert Hoover (San Francisco) -- first campaigns were for State Senate, Congress, US Senate, Governor, and Vice President and President respectively. (Roosevelt and Hoover had held high appointed office before those campaigns).
We certainly see this among the leading candidates for President today. Ted Cruz and Hillary Clinton each made their first campaigns in races for US Senate. Donald Trump has been Donald Trump. Jeb Bush's first campaign was for Governor. The out-of-the-money Democrats (Sanders and O'Malley) are each former big city (Burlington = big for Vermont) Mayors.
You can extend this observation to past unsuccessful nominees of the past 40+ years. Romney's first campaign was for US Senate. McCain's, Kerry's, Gore's and McGovern's were for Congress. Dole's and Dukakis's were for state legislature. Mondale's was for State Attorney General. (The unsuccessful nominees of 1964 and 1968 were both veterans of big city politics.)
All that said, City Hall may not be a path to the very top, but certainly does well. Many mayors among past and present Governors and US Senators. Being a Mayor of a very big city does seem to be something of a political disadvantage, due to perceived corruption / entanglement with / advocacy for disfavored minorities who predominate in big cities. Mayors of New York, Los Angeles, Chicago, Houston, Miami, Boston, Atlanta and Dallas have not advanced to higher office in a very long time (if at all). Baltimore and San Francisco are notable exceptions, producing two recent Governors in the case of Baltimore, and a current US Senator and (likely) Governor in the case of San Francisco.