Do you have access to phpMyAdmin on your host? (you can also install it locally and connect to a remote database)
phpMyAdmin has a neat export tool where you can select a database table and they have a lot of supported formats for export including directly to Excel/Opendocument sheets.
Other database tools probably have similar features.
The Flarum API produces JSON natively but it's not easy to get the full list if you have more than 50 users.
The Flarum admin page and the Flamarkt Backoffice users page each show users in a table where most browsers will allow copy-pasting as a table, but they don't show all the data and some attributes like dates might not be in a format that easily be parsed back to a timestamp.
In both cases it's a challenge to merge relationships, but I assume most of the interesting data would be on the users
table itself.
My Formulaire extension has a built-in export tool but it's not to export users, it exports form submissions and optionally includes all the user data together with each submission.