As opposed to keeping politicians honest when they overpromise and underdeliver? Populism can be a tool for good but even then it is a form of deceit, you're promising more than what you can deliver, and you're not doing it because the promises themselves are good but because making the promises convinces people to vote for you.
Populism is just as bad as, if not worse than, nationalism. Both are simply forms of governance and appeal rooted in unrealistic desires and negative tendencies.
Normally I don't look to the dictionary for arguments, but in this case it seems instructive. You're adding on irrelevant bullshit to a perfectly fine and well understood definition. What you are adding is uniformly negative: "you're promising more than what you can deliver"; "a form of deceit"; "forms of governance and appeal rooted in unrealistic desires and negative tendencies": none of these are in any way necessary parts of
populism
noun
a political approach that strives to appeal to ordinary people who feel that their concerns are disregarded by established elite groups.
While it is true that a politician of any kind can lie about what they will do once elected or misrepresent their capabilities, there is nothing about populism which necessitates
any of that.
You may stop reading here, as the above wins the argument: the following is just elaboration for anyone interested.
There are different kinds of populism; different reasons why various ordinary people feel that their concerns are disregarded by established groups. There are a variety of different flavors of economic populism. There are efforts to incite a new pogrom or divide people against one another or to fear an outsider-- and a common reason for this is that elites of one sort or another have organized such a campaign because they would rather slaughter a scapegoat and hope it satisfies the mob than address their material needs. For example: Henry Ford published
The International Jew in the interwar period which, as you might guess by the title and the interwar context, is a North American short fuckton of antisemitic nonsense. Why would he do such a thing? Materially, any among the capitalist class (or any ruling class, really) have reasons to want to encourage society to do something awful. When there is a movement that wants to do something awful, people of good conscience rightly stand against it. Occupying the time and effort of people of good conscience is very useful to a ruling class for what should be obvious reasons. But it in case it isn't obvious, ruling classes are built on exploitation. That exploitation has material consequences for those who are exploited. The worse those material consequences are, in most cases and other things being equal, the better for those who are doing the exploiting: the less that is spent on the upkeep of those who spend their lives in miserable toil, the better for those that own their produce. And so: persons of good conscience will struggle against that exploitation if they have nothing better that they could be doing. Opposing a genocide or other atrocity is a better use of one's time than the struggle against exploitation (to the extent that such struggles cannot be combined), so a ruling class which presides over a society that is doing awful things has an easier time maximizing the exploitation of its subordinate classes. Also, scapegoats.
All of this has happened before and all of this will happen again. Ford's publication of antisemitic nonsense occurred in 1920, which is around the height of the
First Red Scare in the United States. That's almost certainly not just a coincidence: America's first red scare was a reaction to another very different form of populism: the radical labor movement. There is nothing particularly unrealistic about the demands of the radical labor movement unless by 'unrealistic' you mean that capitalists would rather brutalize people making their demands than accede to them and that capitalists have control of the government and its police and military. In that sense, yes, radicals should expect that the servants of the capitalist class will try to murder them.