Aggie is a lovely nickname for Agnes. It wouldn't be good as a full name, but I like it for short. If I were to name my daughter Agnes, I would absolutely call her Aggie. It's a really spunky, cute name with loads of personality and fire behind it. Besides, I know a sweet girl who is Agnes and friends call her Aggie and she leaves a good impression on the name.
This name is too infantile to be used as a full name. Unfortunately, Agnes sounds ugly in English. I'd go for something else. If you must, name your daughter Agnes and call her Aggie. She will have certain credibility problems if she's a woman named Aggie.