I think the film's portrayal of Arceus was fantastic - they show it as a mighty Pokémon who saved the entire world from certain destruction, and a loving, caring powerful being. However, they severely downplay its abilities. If I had to guess why they downplay its power so much, I would have to say it's because of Pokémon's long-standing belief that "no single Pokémon is the strongest". (And they seem to have a thing with not wanting to have a true "God" of Pokémon, which I'm okay with.) In order to make it appear Arceus isn't an all-powerful god, they have to make it have weaknesses.
One of the most important points they downplay is Arceus' ability to nullify any and all attacks with the Life Plates. Remember, the only reason it was damaged by the betrayal and Pikachu's attacks was because it gave part of its life / power to Damos through the Jewel of Life. With every Life Plate, Arceus would be able to negate any attack thrown at it. You may ask, "but why didn't that work when it confronted the meteor?" The only answer I can give is to go back to one of my first points - they have to show that Arceus has weaknesses.
Another point they downplay is Arceus' ability to create entire worlds - they only mention it briefly when Sheena and Kevin are explaining the legend of Michina to Ash and company. They also downplay the fact it created Dialga, Palkia, and Giratina, who govern such powerful forces. (And they don't even mention its creation of the Lake Trio, though they had nothing to do with the films, so this is understandable.)
In the end, it's very easy to view their portrayal of Arceus in the 12th film as lacking for such a mighty Pokémon. However, this is only because they downplay its abilities so much - it is most definitely a powerful Pokémon.