Atm this limitation is in place as the username directly determines the account file name. See also Allow usernames with capital letters – just as info, not a reasoning.
Edit:
And to give some more context, at the moment Kirby loads and initializes all users by looking at the files in the account directory and registering them with the filenames as their usernames. Only when the actual user data is needed, the file content will be read. Diverting from the concept that username equals (possible) filename would imply that every account file has to be read first to determine the actual username. I’m wondering what this would do to performance. Other thoughts?