Ivy's fate

The giant flower traps her when she shouts "Curses!" and it must have attacked her and drained her of her powers because Ivy appears disheveled and as if she has been violated in some way when she is in Arkham. Freeze might have frozen her to death since looks determined to get revenge on her attempt to kill his wife.

I would have liked her to reform in the end like Freeze and maybe become Robin's girlfriend.

Well, the end of the film shows her in Arkham Asylum with Mr. Freeze. If anything, Mr Freeze will make her life a living hell, but he cannot kill her, or he would not be able to continue saving Nora. Therefore, the most probable ending would be for Ivy to be tormented by Mr. Freeze (though I think for a short period, as Mr. Freeze doesn't seem to be a sadist).

As to whether she deserved it, I'll say both Yes and No.
1) Yes: By the time she severed Nora's chance at life, she had made herself irredeemable. In terms of her action, she deserved it. (Some may also argue that she is irredeemable when she defied God.)

2) No: Pamela Isley's goals have always been subjectively right, and noble in vision. She defied Woodrue's offer of world domination, and after her transformation, she uses her powers to save the environment. Although her methods were wrong in the eyes of society, she didn't deserve the torment of Mr.Freeze.

Ivy seeming to be eaten by a plant but then somehow having survived was just one of the dumb things in the movie.

But yeah, Freeze would torment her but not too much or he would risk getting himself and whoever he bribed in more trouble. I would say she didn't deserve the torment as she really did seem crazy.