I think it is more complicated than that. On the one paw, I think there is some validity in deTocqueville's "tyranny of the majority" argument (which I think RH tacitly espouses). I believe that the main thrust of this argument is not that the masses are stupid but that the very dynamic of populism can make the emergency of a tyranny more likely - if people are not sufficiently vigilant. That is a big "if" I reckon.
On the second paw, mass movements can be easily manipulated by elites toward reactionary ends. Indeed there is plenty of historical evidence from Eastern European pogroms to the cultural revolution in China, Yugoslavia and Rwanda. In facts, there is a far greater chance that a mass movement can turn into a murderous reactionary lynching mob than into a progressive force.
On the third paw, the character of a social movement - whether it is progressive or reactionary depends on a host of macrostructural (economic, cultural) conditions as well as the strength of various elites. The 1917 Russia produced a socialist revolution because the reactionary elements (the army officers = landowners) were in disarray (courtesy of the German Army), and the bourgeoisie not only being weak but also loosing whatever credibility they had by continuing an unpopular war on the request of Western powers.
The 1930 Germany produced a fascist takeover because the elites were strong, and could steer the popular discontent into the fascist direction. Had the German Army officer corps be as decimated as that of the Russian army - the Liebknecht-Luxemburg team would probably replace the Ebert government in 1919.
On the fourth paw - social movement can be a good thing if used properly. Entrenched elites are in a desperate need of a serious shakeup, but social/mass movements alone can rarely deliver. Even a dying regime can defeat them (cf. Saddam Hussein). However, such movements can serve as conduits for powers that are capable of toppling the entrenched elites. Again, Iraq provides a good counterfactual. Whatever one may think of the US invasion of Iraq, it was the lesser evil than the Saddam regime. The US could have easily gained popular support if they mobilized a popular movement in their support. But the Bushies were too arrogant and stupid to do that - and lost it.
So the bottom line is that populism can be and sometimes is a progressive force but more often it is not - and it is turned into a tool in the hand of the elites (e.g. the Solidarity movement in Poland or the cultural revolution in China), a lynching mob (the US) or a genocidal fascist horde (Yugoslavia, Rwanda).
Wojtek