IIRmyancientGreecelessonsC, the answer would be 'no'. Cleon was a demagogue, yet his policies were sounder than the 'non-populist' politicians of the time (Alcibiades and that other dude... again, IIRC). He showed initiative and courage, and I know this because I used the words 'initiative and courage' in my essay on him.
Anyway, if you want an example of populism not always being bad, see Cleon. Usually 'populism' is just code for 'you aren't playing the game the way we want you to'.