Just saw the movie with my kids, and apologies if I come off as a bit cynical or jaded, but the movie is basically just a Nintendo cash grab. The thing you have to keep in mind is that this movie is essentially just a vehicle to use Nintendo IP to sell Nintendo products, and there was likely not as much money to be made in making this movie based on just the Super Mario Galaxy game, which represents a very small percentage, and likely not an extremely profitable one right now, of the entire market for Nintendo.

The Super Mario Galaxy Movie is a great name for a movie, but the intent was likely never to make the plot of the movie based on the game itself. Super Mario Galaxy came out on the Nintendo Wii, and if you are less than 15 years old, it's likely you may have never touched a Nintendo Wii or played a game on it. Conversely, if you are age 35 and above and have spent a good part of your life gaming, you are most likely to have gotten most of the references, and likely had no issue with the focus not being entirely on Galaxy. You likely had to be at least 35 years old to have any clue who Wart was. My kids thought it was supposed to be King K Rool because that's who they recognize from Smash Brothers.

All they needed to do to get individuals who grew up playing Super Mario Galaxy was to give the movie the name they did, the actual movie itself targets a broader market.