Mary Poppins is an interesting character. She’s a nanny who flies, and she seems to be ruled by the changing wind. Granted, I haven’t read any of the books so this is purely based on the movie. I thought that I’d divide this into the two possibilities.
She’s a witch
There’s no disputing that she flies. It seems to me that she traded in her broomstick for an umbrella. She’s full of magic. We normally attribute magic to evil things. She talks to the animals (at least she sings to them). She seems to have some sort of unworldly fascination with a homeless woman’s birds. She can levitate as evidenced by her sliding up the banister. She has to be on some sort of drugs and deal to the children as well. This is evidenced by their shared hallucination in the cartoon. She speaks in tongues “supercallafragilisticexbealadoshus. Is she evil? Most people would say no, but I’m not so sure. She
She’s an angel
She comes when the family needs her most. She teaches the father and mother to appreciate their children. She makes a dull life interesting for the kids. She touches most of the people that she interacts with. She leaves when her job is completed seemingly floating back up to heaven.
In the end, I'd have to say that she's an angel with suspect connections to drugs, magic and chimey sweeps.