To me they seem like two very different domains.
For the most part I feel like it is fairly clear what I should aim for in life to "make the world a better place" - be a compassionate, generous, and considerate person, try to guide others on paths that are wholesome and healthy if my advice is asked, be aware of how my life impacts my environment and behave such that I minimize the damage I do and try to give back... I always appreciate when people say they want to make the world a better place because it implies a big-picture and altruistic focus. I have never really taken that wording in particular as being especially haughty, because it's just a goal. It just speaks to what the person would like to do. It seems to me like there are a multitude of ways someone can help the world in general and all they need to do to achieve that goal is contribute a little in their own individual way.
That said, I do know what you are talking about in your last sentence, and I agree with Z Buck that if they are doing it for less wholesome reasons then it is likely to be tainted. I have also run into a few people who seem very shallow and idealistic like this, but part of it is that they don't like sharing about their own struggles, and so they tend to come off as naive and stuck-up, though from knowing them I learned that they do have plenty of realistic thought and negative experience, but tend to keep it inside.