The machfb is broken in the macppc port.You have to disable it before you can boot into NetBSD.You will get a garbled up screen if you don't. This has only been tested on an Apple Imac G3 333mhz version.
That means it has trouble with whatever graphics chip the 333MHz iMac G3 uses, not macppc in general.
Try to boot netbsd without disabling machfb.
Works fine here.Since you didn't provide any useful detail I can't tell you what's going on.
To figure out what's wrong we need: - ofctl -p output - dmesg output