Actually, it depends on the film. Most sequels manage to do better than the first one, especially in the case of bigger budget super hero movies. Some animated movies manage to get a larger take in their second (and sometimes third) outing.Sequels occasionally stumble badly, even in a pop-culture landscape that gobbles up family-friendly CGI products. I'm still surprised that Happy Feet II flopped (and I say that as someone who didn't really warm up to the original).
I'm personally not surprised Happy Feet 2 flopped. It just didn't look interesting on any level, and The Muppets actually looked like a refreshingly low key film by comparison.