I've always preferred looking at Narnia as encompassing innocence (rather than being 'heaven,' whatever exactly that is), the childlike wonder we have when we enter the world, newly created. And about how we lose that innocence. So saying that Sarah could not return to Narnian because she became interested in 'lipstick and invitations' I think is more of a reference to her losing her childlike interest as she becomes more focused on the 'real world' and its materialistic nature, than any other one. Or, perhaps it is her that she has entered a state of complacency when it comes to the actual nature of the world, and simply lives out her daily life based on more 'trivial' things, which could very well be just about anything we humans busy ourselves with on earth, when we consider the larger picture.
Of course, in using the word 'innocence' I risk so much equivocation that I was reluctant to post.
The other quick point I'd like to make is simply that I believe allegory in itself is only dislikable so long as you read too much of what the author has to say about his/her story, and don't simply enjoy what you'd like to think about it.
-Durelin