Do I need a Mac mini for iMessage support

You need some macOS device signed into Messages. It does not have to be a Mac mini - any Mac works. OpenClaw’s iMessage integrations run on macOS (BlueBubbles or imsg), while the Gateway can run elsewhere.
Common setups:
Run the Gateway on Linux/VPS, and point channels.imessage.cliPath at an SSH wrapper that runs imsg on the Mac.
Run everything on the Mac if you want the simplest single‑machine setup.
Docs: iMessage, BlueBubbles, Mac remote mode.