The fact that Poison Ivy hates vegans and thinks humans should only eat meat is evidence that her goals and motives aren't rational. Her argument is pure emotion, she hasn't thought anything through about what her goals would do to the plants she's trying to protect.

Feeding calories to livestock and eating them is much less efficient than directly eating plants.

this is largely irrelevant, since most of the plants that are fed to animals are crop seconds or parts of plants that we don't want to eat. they eat, for instance, cottonseed, which we grow for textiles, and soy cake, which is a byproduct of soybean oil production.

aside, plants love spreading seeds. they probably think it's better to grow more of them.

but would a talking plant have enough intelligence to think of that? couldn't they even go as far as to hate the plants commonly used as food and thus like that more of them are being eaten? depending on the type of plant they are, the death of other plants could mean that the type of plant that the talking plant is could have better or longer lives after the death of those other plants