Except that they are only true when they are true - which is the same as stereotypes in the real world - some asians are indeed good at math, but that doesn't mean the stereotype is valid.
And given that (as noted above by radja) undead such as ghosts are not always evil, one can't say that undead are always evil.
And (as noted above by Unwise) it depends on the campaign. In my campaign, the undead the party has encountered have definitely been evil, but they may well be necessary allies agains the demons - in "grey" campaigns such as mine, just because something is evil doesn't preclude it from being your ally.