Flarum usernames can only be alphanumerica (A-Z, 0-9). So if you're converting from a site where usernames allow more characters, you're going to run into these issues.
There's an open issue on Github about having usernames that support more characters (UTF-8), so Flarum is aware of this. At the moment, no, mentions won't work with those usernames and you'll probably want to have them changed.
You can check out the Nickname Changer extension for a workaround for your users at the moment.
