No, there have been some great zombie games recently. Left 4 Dead is awesome, Resident Evil 5 was great, and Dead Rising 2 looks pretty good. I think the zombie fad is great because developers can be creative how they want to implement them, unlike the WW2 fad,; there's only so many different ways to play through the same war.
It's just a way to appeal to younger audiences. Some parents might be reluctant to get music games with songs like Down With the Sickness and Chop Suey. But if it has a friendly Lego face it will be all right. I'm still not sure if it is worth it's own game. If has a decent setlist with a good amount of songs it will just be a good track pack.
They don't need to. They'll still make money off of both DS and DSi either way. Unfortunately, I don't see them making a Mario or anything big for it. So "hardcore" gamers probably aren't going to all upgrade quickly. But, on the other side, the indie market will make some people interested.