I suggest that it is impossible to provide a single definition for "populism." There are family resemblances between the great numbers of descriptions, but that's how far it goes, especially, if there is an attempt to attach a value judgement to it (which almost always the case). 

One could, for example, say that the first Christians were populists. The Christian message was a simple message (a simplicity that authors such as Origen and Alexander of Lycopolis considered a virtue). Its leader was charismatic and anti establishment, and most of his followers were simple, poor and unimportant. Does this make "populism" a Christian virtue?