I live in the U.S., and I've never heard "populist" used pejoratively. Where are you from and what does it connote when used pejoratively in your country?
Europe. It usually means when one tries to grab votes at the cost of doing what's "correct" from one's point of view. A stereotypical case would be providing "bread and circus" in the short term while the country is sinking in the mid term.
The following headlines are example use of "populist" with negative connotations (they are not the only ones, I just provide anecdotal evidence of the common use of "populist" as a negative trait).
Here in Europe it's used to refer to politicians who pander to the sentiments of the common people, without having a well thought out long-term strategy; e.g., "I'll lower all taxes AND give free healthcare for all."
The following headlines are example use of "populist" with negative connotations (they are not the only ones, I just provide anecdotal evidence of the common use of "populist" as a negative trait).
British English:
http://www.bbc.co.uk/news/world-europe-17837098
http://news.bbc.co.uk/democracylive/hi/house_of_lords/newsid... (beware starts parliamentary video automatically)
http://www.guardian.co.uk/commentisfree/2011/aug/23/punitive...
French:
http://www.lemonde.fr/idees/ensemble/2012/02/09/le-populisme...
Spanish:
http://www.abc.es/20120515/opinion/abcp-populista-eres-20120...