Populism is definitely a part of democracy, but it is a criticism from "responsible" politicians for "irresponsible" ones.
Obviously this is all politics so you needn't worry about the specifics of what actually is populist.
But, imagine two "responsible" politicians.
One who believes in lowering taxes as a worthwhile thing, and acknowledges cuts to services as a negative impact that is outweighed by the good.
The other believes in higher public spending, with the negative being higher taxes, outweighed by the better services.
Both would be angered by a third candidate that came along promising both lower taxes and higher public spending - just the "popular" parts of their respective manifestos.
Potentially - it's not like a strictly defined term. With mainstream political parties you'd more often think about specific policy areas than the whole business.
I’d respectfully submit that the reason it’s not well defined is that it’s a pejorative deployed by the establishment in both the left and the right to disparage substantive policies they don’t like, rather than a principled label for a particular political approach versus another.
Obviously this is all politics so you needn't worry about the specifics of what actually is populist.
But, imagine two "responsible" politicians.
One who believes in lowering taxes as a worthwhile thing, and acknowledges cuts to services as a negative impact that is outweighed by the good.
The other believes in higher public spending, with the negative being higher taxes, outweighed by the better services.
Both would be angered by a third candidate that came along promising both lower taxes and higher public spending - just the "popular" parts of their respective manifestos.