Maybe it would be a good idea to tell your niece that you're okay with her not loving the quilt and at the same time explain that you made it out of love for her, and mention all the nice thoughts you had about her as you were coming up with the words for each letter of her name. If a lack of empathy is part of what defines Asperger's, then she may need a little spelling out about what making the quilt meant to you. Who knows, when that hits home, she may immediately feel more receptive toward it. The other reason that it would be good to give her a little explanation would be to assure her that whatever she thinks of your quilt, you'll always be her loving aunt. Some of the other young people and maybe even her mom may have shamed her for her reaction as soon as you were out of earshot, and that would be an unintended side effect. It would not make her feel any more kindly toward it either, if that happened.
The other thing to know is that if the quilt had been given to her in a less distracting situation, she might have been able to focus on it differently. A big social gathering is often stressful for people like your niece - too much noise and activity. She may be loving it by now, for all we know. It wouldn't surprise me.