How so? She kidnaps Rapunzel at an early age, locks her away for nigh on eighteen years, refuses her any sort of regular teenage life and basically only holds on to her for her power. Do you honestly think Mother would have Rapunzel if she was powerless? No. Mother didn't love Rapunzel, she loved Rapunzel's magic. Mother's portrayal and exit are justified.
Disclaimer: I enjoyed the film.
The Mother, who is clearly a Gaia-esque figure, finds a way to retain her youth by living off the land, so to speak. She does this by finding a special plant; hers by right of the age old Finders Keepers Law. She does so for hundreds of years and in a sustainable way. Nothing inherently evil here.
The King has it found, uprooted and boiled to save his sick wife. Doesn't ask, just assumes it's his because he's The King and he gets to act like a dick by right of birth. He also ends up effectively destroying the plant in a single evening because he doesn't give a shit about sustainability or the environment because he's a dick.
Baby girl is born with magic hair.
Mother Grothel puts two and two together and realises that the magical properties she has carefully harvested for hundreds of years now reside in the girl's hair. Since the King disregarded her rights, she does similarly by breaking in with the intention of taking some of the magical hair. No malice there, just an oppressed citizen fighting for what is rightfully hers. Obviously, this doesn't work out and, in a silly and desperate move, she yoinked the little girl. This was a silly and selfish move, but a human one. She clearly wasn't thinking straight. We've all made mistakes, right? So we can at least sympathise with Grothel even if we can't condone her one silly mistake.
Regardless of her motives, she raises Rapunzal for eighteen years. Raising a human being to be a naive but clearly well adjusted young girl takes a lot of hard fucking work. We're talking nappy changes, teething, feeding, hormones - all that malarkey. Eighteen long years. Then, within 3 minutes of finding out Grothel isn't her real mum, Repunzal throws her, and the eighteen years she spent looking after her, under the bus (not literally, of course).
Regardless of Grothel's feelings for her (and I'd argue that spending eighteen years nurturing her in such confined quarters would naturally cause her to develop at least some maternal love), you'd think that Repunzal would love her since she is the only mother she ever knew. Nope. Spoilt little bitch pushes her out the window the minute she finds out fame and riches await.
So yeah, I do think Grothel got a hard time. Ugly people nearly always do in all Disney movies.
Also, Cyanide: it's a cartoon. Your definition (wherever you picked it up from) is extremely narrow. Where a film isn't 100% CG, it isn't a cartoon, just like Who Framed Roger Rabbit isn't; it's a live action film with cartoon elements. There is no argument to be made.
EDIT: Polar Express is most definitely a cartoon. It used an essentially modern version of Rotoscoping which was a technique used in Snow White.