Populists speak in a language that people understand, and don't obscure their policies in meaningless jargon. This simplification does not mean that their ideas have no weight.
A populist gains their power first and foremost by connecting with people and engaging them in a political process they would otherwise feel alienated from. If they can cut to the heart of the matter with a solution that is simple and effective - why penalise them.
Secondly though, when people vote for populists they also evaluate the merits of their policies. Even if those evaluations aren't technically proficient, they are important because people don't need expert knowledge to know what will be best for them. There's a reason we elect our officials rather than choose a panel of experts to run the government - because we think that regardless of their knowledge, experts do not have access to our interests and feelings.