Integrating better with third party app

You'll be able to create users just fine if you need:
https://api.mattermost.com/#tag/users%2Fpaths%2F~1users~1create%2Fpost

One good thing of mattermost is that everything that is done in the user interface (from creating channels to creating users, sending messages, getting notifications, etc) it is all via APIs, so you can build your own UI if you wanted to.

Good luck!