Not necessarily. It depends on context. For example, if I'm talking about myself after a very indulgent meal, I might say, "Man, I really pigged out on those tacos." However, maybe my wife is angry that I went back on my diet and says, "Stop pigging out on chocolate all day! It's unhealthy!" In this case, it might be a bit more insulting. Or if you're watching a stranger and the airport and judging their physique and eating habits, it might be pretty negative-sounding. It's all context and who you're speaking with.

It is most often used with the pronoun "we" or "I." It is an informal expression. It means to eat too much, to eat rapidly, and, often, to eat junk food. Obviously, it is not insulting when you use it about yourself. It is a humorous confession. The only meaning is to eat a lot, too much, gluttonously. There isn't any other hidden meaning. Alex's examples are perfect.