using your same analogy of cake/icing: the cake is the 'meal' with the icing being the 'accent'. If the cake is CRAP, all you are getting is sugar coated crap!

A stable, secure backend that is well documented is what people want first. Anyone can build a GUI onto that once done right: phones, websites, other businesses (if in the CORS whitelist), etc.

But if you do not have a stable AND secure backend, you are just adding a pretty face onto a crap backend. Sure it looks nice for a little bit but any REAL USAGE of the tool and suddenly you lose users PERMANENTLY!