Yes, that is the only way to "use" Kodi on a Roku, by using the mirror function. Be aware that Windows computers are very difficult to mirror onto a Roku. I believe you have to have Windows 10, and then there are some hardware requirements as well. I've never been able to mirror my PC to a Roku, although my Android phone does it fine. Apple products also do not mirror or cast to the Roku. The most reliable method I can think of to use Kodi and view it on a Roku would be to use an Android tablet/phone to connect to Kodi and mirror it to the Roku. Not the easiest method, but in my opinion the most reliable method.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.