IIRC you can choose your monzo.me address, and it doesn’t have to be your name, although that’s preferable for the sake of clarity. You could change it to be monzo.me/squif if you wanted, you just have to contact monzo.
With the example of John Smith, of which there may be many, there’s only so many variations before you have to chuck in extra numbers and letters to make the address unique. If monzo.me/johnsmith321 is allowable, then anything should/could be.