Full warning: I loved Mary Poppins Returns (though that is likely because, as a Brit, it is my childhood and because I also adore Emily Blunt), and it took the number one spot on my favourites of 2018 list -- which tends to be reserved for movies that me feel extremely happy over anything else, e.g. Paddington 2 or La La Land. So I may be just a little biased...
I think there's enough of a storyform to identify the major 'notes', but I think you're right -- it's more about bringing joy than telling a complete story. The OS is the most obvious Obtaining story ever ('Get the Money to Save the Banks House'), the IC literally flies in to influence those around her to live a little, etc.
But I mentioned something in the Dramatica Discord that I found fascinating, which is that MPR has what appears to be an MC hand-off (which I don't ever recall seeing before). It's introduced as Michael Banks who is grieving the loss of his wife and has become 'too grown-up' in his approach to life, and all of this then shifts to the kids for the majority of the movie (who similarly have become too 'intellectual'), before shifting back to Michael right at the end. So you have this arc that they share of characters who start off 'too grown-up' and eventually learn to embrace their inner child.
So, to that extent, there is enough that I can identify to make me think there might be something there -- especially since the movie clearly makes an argument. But I don't know how much of a storyform is there. I just can't wait to watch it again, I loved it.