Yes, mouse and keyboard are supported on PC (Windows / Linux) and Mac for games that accept those inputs.
When you launch a game session on a compatible device, your mouse and keyboard inputs are automatically passed through to the game. This works just as it would on a regular gaming PC.
Note for PC users: When a game session is running, cursor lock is active — your mouse is captured by the stream so all movements go to the game. To release your cursor, open the In-Game Console Options or exit fullscreen. Also be aware that Ctrl+W (Windows) or Cmd+W (Mac) will close your browser tab — avoid using W as a movement key near Ctrl/Cmd, or use a controller instead.