There is a desktop application version of Joplin and a command line version of Joplin. The script that you ran above installs the desktop application version of Joplin, it is not run by invoking the joplin command, instead you should use your OS's default application launcher
The script downloads an AppImage and in most cases creates the shortcut as well. Some distros require the user to log out and back in again before the link to the AppImage is recognised.