More specifically there can be no separation between how something looks and how it works, they're bonded (and for good reason). There is no scenario where you can actually fully break the two apart. Any attempt to try harms one side or the other or both sides. How it works and how it looks go in tandem in all cases, they heavily influence eachother, either making the whole better or worse depending on how you go about creating for that reality.

Anybody telling you different is arguing from a pretty severe bias. If you're designing on the basis of how something works, and not also on how it looks at the same time, you're going to create a subpar outcome.