agree with bootmii's definition
I see it as a problem because the purpose of any ruler-selection system (like a democratic government) is to select a good leader, and broadly speaking, IMO, the average person is terrible at recognizing leadership qualities in a candidate and electing based on that. Populism tends to lead to people being selected based on how rhetorically in-tune they are to the temperature in the room. At best, if the candidate is actually in-tune with that temperature, they meet one of several criteria for being a good leader; even still populist sentiments tend to cloud the selection process on the whole, by way of promoting this one criterion to the deficit of others. At worst, if the candidate is not actually in-tune with that temperature, and only matching it in rhetoric, you end up selecting a candidate on no particularly sound basis of leadership.
At the same time, though, entrenchment in the political class creates its own inefficiencies in leadership, and its own cronyism. Just because the average man is terrible at selecting a leader doesn't mean the members of the elite class are any better. They tend to be very good at selecting people who will maintain their own power; whether they actually select good leaders is largely due to circumstance rather than design. Populist sentiments lead to upsets in the established order, and so act as a necessary check or balance against entrenchment among the elite.
Populism is to politics much like a Louisiana summer afternoon rain is to our local ecosystem: fleeting, fickle, and frustrating to get caught up in, but crucial to maintaining a healthy system and to avoiding a slow, dry suffocation of life and growth.