It depends on what you think Good and Evil are. To some, they're just a label. To others, they're planes with objective sources of power. And even others see them as concrete moral imperatives.
If you know what you think Good and Evil are, use that as guidance on whether undead can be non-evil. If you haven't figured out what Good and Evil are yet, then you have other questions to be asking yourself.
Remember, dude, it's your game.