Simply defined, populism is the rebellion of the common man against the outsiders. This vague definition reflects the reality that there are populists of numerous different political persuasions; at its heart, populism is a strategy, not an ideology. Populism is dangerous because its antagonistic framework prevents proper dialogue between different groups; to compromise allows a morally inferior group to force its views on the people.That's a bit tricky because the definition isn't what it my first seem to be. The point the writer wants to make is that "populism is a strategy, not an ideology". Populism, to flesh it out, is a strategy that stirs people up to think that they are being exploited by an elite group. And you can go with that if you want. It has consequences, though.
The first consequence is that there is, on that definition, no good populism. All populism is bad. The second problem is that there is already a perfectly good word that does what we're told "populism" is and that word is demagoguery. Of course, that is a much harsher word. No one calls themselves a "demagogue" while they might call themselves a "populist". The Acton definition, then, is just a move, a dishonest move, to start a hit job.
I quite like the Acton Institute and read their stuff regularly. I was a little disappointed to seem them stoop to this sort of writing. At the same time, I wasn't surprised. This is a Catholic organization and such organizations have very little optimism in people's ability to make decisions about their lives.