Potterism is Macaulay's word for "the mixture of humbug, sentimentality, commercialism and genuine feeling" which doesn't otherwise have a name but which one recognises as a thing which should have a name when one encounters it in the pages of the Daily Mail. I'm sure she hoped her coinage would stick (and it may have had a bit of slangly currency for a while) but in the end the book that launched it wasn't strong enough to hold it in the public mind and keep it there. This is a pity. It would be good to have a single word with which to pin the thing down.
Untypically for Macaulay it's an angry book- with something that feels a lot like self-disgust taking the place of her usual detached amusement. It came out just after the First World War which may be the explanation. There are few characters to like. After a while it turns into a sort of half-hearted murder mystery.
Potterism the word didn't catch on, Potterism the book is a relative failure (though very readable), but Potterism- the thing itself- flourishes over these defeats in unknowing self-delight.