Q: Where does the Jokermobile fit in Batman’s canon? Is it a necessary piece of their rivalry? — @thybmb
A: Okay, the way I see it, there are really two questions here. The first is a pretty easy one, too: No. Strictly speaking, the Jokermobile is by no means necessary, and there's nothing that it adds to or illuminates about the enmity between Batman and the Joker that you can't get elsewhere, especially when it comes to characters who are built far more explicitly around the idea of mimicking Batman's approach to crime fighting.
The second question is one that's more implicit in the fact that you asked: Can I write an entire column about how the Joker used to ride around Gotham City in a car with his own face on it? And, c'mon. It's me we're talking about here.