Yeah, Joker and Batman have matching themed bases so it makes Ivy look a bit weird for now. She looked out of scale in the first pic of the 3 together in her gallery, but now we see it's just because of her smaller base. I think it'll work out in the long run as more of his characters get different bases of different heights. I like Batman being higher, looking down at his villains, while Ivy is more "grounded" as she should be.
The cool thing about Batman's very eccentric villains is you can have their base really match who they are instead of just putting them in a random environment, or put them on the theme base established by Joker and Batman. They're going to mix very well in the end.