Yes, there is such a thing as „clean“ user interface. User experience has its own organising principles and design heuristics - many of which echo those for internal design (e.g., UI‘s should be as simple as possible, UI‘s should be easy for the user to understand, etc). But as we bolt on more and more features, there‘s a tendency for entropy at the UI level to increase. And so very few teams do on-going work to minimise that entropy. And so, the user interface just keeps getting worse - more complicated, less intuitive, clunkier, less responsive, and so on.