Poison Ivy has to have the dumbest villain scheme in any Batman movie
Her plan to "save" the Earth was to A) unleash an army of genetically modified invasive plants that would inevitably kill all native flora and B) cause a new ice age that would most certainly kill most if not all animals.
Basically, she would've created the worst environmental disaster in history. And while it may have made sense that Mr. Freeze would want to help her takeout Batman, but why would he be complicit in her plan to destroy the world?