why won't it look good ??
Because it will be stretching the resolution.
The Wii only outputs picture at 720x480. The monitor of the Apple 30" is something like 2560x1650.
If you hook these up together, you will get poor picture quality because the small picture produced by the Wii is going to be stretched across that huge monitor.
Try on your Mac. Put your resolution at 800x600 and see how terrible the picture is.