"Populism is a political doctrine that appeals to the interests and conceptions (such as hopes and fears) of the general people, especially contrasting those interests with the interests of the elite."
Democracy: Low taxes on essential products
Populism: High taxes on essential products, then "stamps", "aids", or low taxes/subsidies on a very specific range of said essential product
Also similar to the Roman Strategy of Bread and Circuses
Democracy is a system of government, populism is a strategy for maximizing popularity by appealing to emotions with rhetoric and unrealistic proposals, and pitting a group of people against another group. Since I can see where you are going with this, yes, the notion of populism does require the politically incorrect idea that not everyone is perfectly rational, intelligent and well informed. One can be in favour of democracy without necessarily agreeing with the voting decisions of general public, and without agreeing with political strategies that exploit this.
(just like one can be in favour of the freedom of speech to say X without agreeing with X)