Is the only way to sideload the mobile app?

Not until Hulu updates their software for Android TV.

You can use casting from Hulu app on your phone:

https://www.reddit.com/r/Hulu/comments/8xkltg/why_doesnt_hulu_live_support_android_tv/e243k5w/

Hulu is such a weird company. It doesn't bother to update its app on Android TV. Yet somehow it updated its app on Xbox 360 last year with latest UI and support for profiles and live TV (Seriously, I am not talking about Xbox One). So go figure.

From what I understand, sideloading it doesn’t work. I don’t know why they haven’t released an Android TV update. Can’t be that much different from Fire version.

I sideloaded the phone app and it works pretty well with the stock remote. Not perfect but decent.