I agree with Algalon.
Luffy and the Strawhats are an active-neutral party (ooh, let me explain). They do not meddle with political issues, economy, corruption, hunger, etc. Any problem a person or a country, or even the whole Government and world, are not their Business. Luffy is in the pirate business in order to get as much freedom as possible (refer to Luffy and Rayleigh convo, in the Sabaody Archipelago). And that is a clear distinction between a Pirate and a Revolutionary. The pirates are selfish and work for their own good, and freedom, whereas the Revolutionaries work directly to overthrow the Government and fix the World.
Now, of course Luffy is a good person, and so are the Straw Hats. They make friends, and some of those friends do insert themselves in those types of situations. Vivi was the princess of Alabasta, so, in order to help her, Luffy meddled into Alabasta's business. It has been this way for a long time, and will be like this up until the very latest Manga chapter (and will most likely continue). Further on, in the Fishman Island Arc, Luffy will spout a very interesting analogy that sums this up, you'll know what it is when you read it.
There is a progression, though. The last arcs have been quite different from the norm. But true change won't happen, in my opinion, until they meet Dragon, who will explain to them what the Revolutionaries fight for, or when Robin herself explains what's happening, after discovering the Ryo Phoneglyph and the True History of the Void Century. That's when Luffy will snap out of the good-hearted but "selfish" mind he that only helps when his friends are need (trust, he does have a huge amount of them, and every country he steps in comes out better when he steps out).
Then again, he doesn't like scaredy cats. He doesn't help people that show no will to fight, no determination, no anything. And most of the world does live in ignorance and a sense of apathy that doesn't go well with Luffy. I think that would be something interesting to see when he finds out the truth about History, or when Dragon shows him the Revs. He looks and sees people with no determination, even if pleading, and will choose not to help them. And that is quite powerful, people can't expect everything to work out, if they don't work towards it. Luffy would have never helped Alabasta if only he had seen was people dying of thirst. He helped because Luffy poured her heart out in helping her country and was repelled, and the old man from the oasis worked his whole life to revive the oasis and had it taken away by Crocodile once again. Those are the kinds of acts that make Luffy become friends with them, and makes him help them in return. And, to be honest, that is something I really agree with in One Piece. And all this, despite being said in a very childish way in the beginning of One Piece (refer to Luffy's convo with Coby, in the very first arc of One Piece), but has been felt throughout the whole story, and, only if you stop to think about it, will you find the maturity of the motto behind the childishness.
Tl;dr : Anything can be has childish or as mature as the reader of the work.
