I think your question conflates design and development, though they’re not even close to the same thing. Sites are rarely “designed from code”…they’re designed in Photoshop by designers and translated into code by developers (of course, there are exceptions to this, but it’s usually the case).
But I’ll assume you’re just saying, “since WordPress does a lot of the work for me upfront, do I really need to know how it works?”
WordPress is a great framework to build on top of, but the system itself is made of “raw” css, php, html and JavaScript. If you don’t understand all of these things (at least at a basic level), you’ll never be able to develop your own themes and plugins, or even to customize existing ones. And in general, you won’t be able to translate finished designs to functioning sites.
Because WordPress is so full-featured right out of the box (and because awesome support communities like this exist!), a lot of non-professionals get by as “copy-and-pasters”; never really understanding the code, but able to hack their way through issues as they pop up. This is fine if you’re running a single, personal website. But if you plan to develop WordPress sites for other people (ie. professionally), you cannot get by without a solid understanding of php, html, css and JavaScript.
In fact, there’s even MORE stuff you need to know. SQL, XML, JSON, AJAX, http protocol basics, apache configuration, SEO techniques, basic *nix commands, domain configuration…the list gets longer every day, not shorter.
So in short–yes, you need to have at least a basic understanding of how all of that stuff works if you plan to make websites for a living. No WYSIWYG or RAD tool will (in the foreseeable future, anyway) come along to replace the expertise of professional web designers and developers.