I can use the Home app and AirPlay with Roku, so I would think it's possible to connect a HomePod mini with it as well. Mine lets me turn it on and off at least, but I don't have a lot of control within Roku.
https://blog.roku.com/apple-airplay-homekit#!
It looks to be for 4K devices only, so if you have an older 1080P, you may be out of luck.