Well, funny enough, I am one of those guys who is equally comfortable doing mechanical, electrical and software engineering. In all cases, the "beautification" of the product happens after the underlying principles, data structures, circuit fundamentals, loads, thermal requirements, etc. are well understood and have gone through many prototyping and testing phases. I am not talking about minimum-viable-product stuff here.

Yes, if the goal is a minimum-viable-product type web solution, by all means, make it pretty and fake the rest until you see enough traction to figure out if there's a real business.

For nearly anything else, the internals need to be well understood before it makes sense to do anything else. I can't think of one mechanical design I've done where I spent a ton of time figuring out color, size, shape and location of buttons and knobs before fully understanding what needed to go inside the box, what the electronics was going to look like, communications protocols, power supply requirements, environmental requirements, etc.

So, yes, if I was going to design a car I'd start by selecting an engine, drive train and suspension components and then designing the of the vehicle rest around it. You'd fit artists concepts and renderings to the realities of the underlying mechanics. It then becomes an iterative process where you push and pull and make adjustments to both the artistic expression and the technical realities of the design in order to converge on a product that can be released.

Let me state the obvious: These analogies are all imperfect.