The problem with that is that “pretty” is implying simple visual beauty. It diminishes what design really is: the way something works not simply how it looks. “Pretty” should be integral to “make it work.” If your design is good, it’s more self-documenting, so pretty early in the process means less work documenting — it also reveals more of the essence of what exactly you are building which then helps drive the decisions on what to build next.

Design shouldn’t be an afterthought, it should be THE thought.

I disagree. If you make something, THE thought is that it fulfills its function correctly. I wouldn't pursue an excellent UX if the inner workings are buggy.