Theme is HUGE to me. I won't buy a game if theme sucks no matter how good gameplay is. On the flip side, I won't buy a great theme with crappy gameplay. There has to be a balance.
As for theme getting kids interested in Pinball, let's be honest. The only kids these days that will ever be interested in Pinball are pinheads kids. Kids that have machines in their house to play and learn. Most kids don't have this luxury, and without arcades like the 70's-80's, there just isn't any exposure to them like we grew up with.
Would a crummy Minecraft pinball appeal to kids now. Probably, but only briefly. Pinball has to be geared towards adults and or entire families the sell. I would never buy a Minecraft pin, even if my kids wanted it because the novelty would wear off, and I have no personal interest in it.