This isn't really an issue with laravel mix as it is intended to handle the "80% use case" scenario... This falls outside that 80% because tinyMCE apparently isn't quite compatible with webpack -- at least, not without some extra webpack config to make things work properly.
See: http://stackoverflow.com/questions/41009736/tinymce-uncaught-typeerror-theme-is-not-a-constructor
Not tested, but you ought to be able to add that necessary extra webpack config with mix.webpackConfig() as documented here: https://github.com/JeffreyWay/laravel-mix/blob/master/docs/quick-webpack-configuration.md