I don't think theme is irrelevant (it can and does add to the experience), but it is not a deciding factor for me, and I am surprised by people who say "I will never by Game X because that theme doesn't do it for me." Especially when they've never even seen the game (say, it doesn't exist yet). Same as saying "I will definitely buy Game Y because I like that theme!" Neither makes sense to me.
I can give examples; I'm sure others here may have different ones. X Files and South Park are great tv shows. And crappy pins. Do I love the theme? Yeah. Did the theme make the pin better? No. Would I ever own either game? No. I wonder if anyone here swore they'd get one of those games (or another) because of the theme, and then changed their mind when it turned out to be a lame game.
The opposite holds true as well. I'm not a LotR fan--never read the books, haven't even seen all 3 movies (none at the theater). Neutral/disinterested on the theme. But the pin looked interesting, and when first I played it 5-6 years ago I really loved it and wanted to own one someday. It's about the *game* and the *experience*, not whether the theme appeals to you. A painting or sculpture can be selected based on how it looks alone (the 'theme' as you will), but a pinball machine is a game. It's playing it that counts, not how it looks sitting in your game room.
Transformers is a decent theme. But if Stern puts out something that's just another rebranding of Avatar/Iron Man (and I truly suspect it might be), or that plays like a crapfest (X Files), do I want that game? No. Am I a huge WOZ fan? No. I think it's a very recognizable theme and brings back nostalgic memories. (Proof of this is when playing one of the different WOZ slot machines out there.) Certainly not an offensive theme, and it's family-friendly for sure. I have no WOZ toys or memorabilia at home. But if it's a solid, interesting, fun game, then I will have one. (I suspect the odds of this are MUCH higher than the odds that Stern will produce another engrossing A-list game soon, with or without SR.) The WOZ theme is certainly not a turn-off or dealbreaker for me.
The other thing I don't get is the obsession with the Muppet theme around here. Do people not realize that the Muppets is basically Sesame Street? Would you go ga-ga over a Sesame Street themed pin? Or think it is too childish? I think the theme is a bit juvenile myself. But if it were a really fun game, I would play it and consider owning one, regardless of how I felt about the theme alone.
I guess everyone is different. But for me, pinball is about the game. The theme can help (if well-integrated) but it doesn't kill a game for me. As an example, Shadow is a very good game. The fact that some people don't like the game doesn't make it less fun in my eyes. I'm not a soccer fan but I will *definitely* own a WCS some day. Etc.
Now, if there was a Gumby or Beavis & Butthead game produced, I would seriously give it a long, hard look. But any purchase decision would rest on whether or not it is an exceptionally fun game.