She doesn't have a heart on her back any more. They changed it.
I understand why Jumba would make something to turn experiments evil. That almost makes sense. Because when Jumba was making his experiments, he didn't know about experiments being converted to good. After it happened to Stitch, he used his new experience on the subject to make 627 impervious to good.
That's the first problem. The second is that Jumba never needed to make an experiment in the first place. He could have made a machine that did it for him - it's been proved that it would work.
Thirdly, all the experiments try to accomplish their function in the most direct possible manner. If, after overcoming those two plot holes described above, Jumba had decided to make an experiment to turn others evil, he would have made one that did it directly. No poncy singing crap. Just something that could fire a laser that changed somebody evil. Hamsterviel managed it.
Finally, I have a problem with her being a pink Stitch clone. There's no creativity there. The only bit they made up was the singing, which was just the most feminine function somebody could come up with. Obviously Stitch can't fall in love with anything that isn't pink with huge eyelashes and pronounced chest with a voice so high he's lucky to be able to hear it, even with an increased hearing range. Apparently this is some guy's idea of what makes something female. I think it's downright degrading.
Angel doesn't even have a personality, she doesn't even say anything. How are you supposed to like something that doesn't talk? The only way Stitch and Lilo bonded was through being able to communicate.
After all that, I hate the way she's turned into a mary-sue character. All Stitch does is think about her, Lilo mopes about her, every character is focused on her... and then she 'saves' everyone. And Lilo is suddenly fine with it. And in Snafu? She sits around for months being trapped, playing some sort of 'damsel in distress', waits for Stitch to show up, and what happens? She saves everyone again, of course!
I hate how this annoying, personality-less, walking (barely) talking cliché turns up, and we're all supposed to be fine with her suddenly jumping into Stitch's life. I despair for the future of animation, I really do.