HomePod – How to Use HomePod Mini as a Mic for Mac Mini

No. This cannot be done.

The HomePod does not have an HDMI interface (HomePod Technical Specifications), so consequently, this would be physically impossible to do.

In fact, even Mac's with an HDMI interface don't support the CEC protocol.

Incidentally, if you want to connect your HomePod to your Apple TV, this is possible as a (limited) control and as an external audio device. The latter is self explanatory, so let's look at the former use case as detailed in this article.

When you connect your HomePod to an Apple TV, you can control it only minimally, such as to pause, resume, and adjust the volume. (You can’t even turn off your Apple TV.) For more complex voice commands, you must use the Siri Remote.

^{Emphasis Mine}

If you want to send complex commands to your Apple TV, you'll need the remote (at least at this time).

You can surely do it with the Shortcuts app. It's available on iOS and every shortcut you create will be synchronised with your other Apple products on the same iCloud account.

It works well with HomeKit, the default weather app and the Apple Music radio stations. You can trigger a shortcut with Siri, and you should check out the user guide for more infos.