Mstdn is a desktop application based on the mobile version of the Mastodon page and the Electron framework. It basically uses Mastodon's mobile page and provides various desktop application features (such as desktop notifications, keybindings, and multi-account support). Mastodon is an open source project. So if you want to make a new UI, you can just fork the project, implement your favorite UI and host it on your place. Then you can participate Mastodon networks from it.