No one should really be writing UI code by modifying a file by hand because UIs are often complex and deep, so the file structure used to store the UI data doesn't need to be human-readable.
This is true for any data - once you hit a moderate level of complexity neither JSON or XML really work - you need to build a tool to manipulate the underlying data rather than changing it directly. If you do that then it doesn't matter how readable or 'ugly' the data is because a human being shouldn't need to access it directly. This leads to other benefits you can optimize for like size or parsing speed or redundancy instead.
This is true for any data - once you hit a moderate level of complexity neither JSON or XML really work - you need to build a tool to manipulate the underlying data rather than changing it directly. If you do that then it doesn't matter how readable or 'ugly' the data is because a human being shouldn't need to access it directly. This leads to other benefits you can optimize for like size or parsing speed or redundancy instead.