As you've noticed, the presented way of enumerating mocks is problematic since you can't exceed a certain number of mocks on the same level of structure, and omitting zeros at the beginning of the ID would cause sorting problems.
Since you already have something around 100 mocks, the simplest solution is to (preferably automatically) add a dash and another 0 before each next digit of your ID; so it would come up with something like that:
02-01 User Settings
You can create as many tree levels as you wish and also, you can create up to 99 mocks on the same level.
For the next time, I'd recommend using folders, so it would look like that:
[Another tree level]
(square brackets represent folders)