+1 to make it the same effort as mod/*/tests/generator. The UI for the actual generation of the random data should be a contrib plugin, there is no need to make it part of core. I wouldn't even be interested in UI, just a CLI interface, so I can automate it easily.
The framework is there but there is a lot of work for core modules to support it. Most of the modules don't have anything under mod/*/tests/generator and those that do - only support basics, e.g. I can see that quiz module can create new quiz instance but generator can't fill it with random data (random questions and then random answers for user's data).