Theme matters to me in that there is a minimum standard that a pinball machine must meet. I care way more about gameplay, but there are certain themes that I would actively dislike and not allow into my house. For me personally, that theme is usually horror or things that are way too adult oriented. Theme has the potential to help a machine in my view, but it has the ability to torpedo a machine more than any other single factor.
Machines that would never come in my house no matter how much I like the gameplay.
Playboy
Nightmare on Elm street
Halloween
Big Juicy melons
Primus
WWE
WWF
The Shadow with original translite (I really hate Alec Baldwin and yes I know it's a lookalike).
Rob Zombie
Alex Cooper Nightmare Castle
Goldeneye (rare wife veto)
Fire and Brimstone
Walking dead is the closest, but my kids wouldn't go back in the arcade with one. Elvira is borderline.
Other folks may love these games and themes and I hope they continue to make a variety of themes for everyone to enjoy, but the theme alone kills it for me on these machines.