You can see her in Ariels's Grotto at Magic Kingdom.
This is from allears.net.
Ariel's Grotto -- This colorful grotto out just beyond Cinderella's Golden Carrousel is decorated with starfish, coral, and waterfalls, and affords the opportunity to meet Ariel, the beautiful mermaid heroine of "The Little Mermaid," throughout the day. There's also a play area with a spongy flooring -- originally there were squirting fountains here, but they haven't operated for some time.
Since mermaids have difficulty walking on land, Ariel will greet you from her chair draped with seaweed -- young mermaid fans can sit right next to her for their photo op!
Ariel's usually there at her grotto from 10 a.m. until park closing, but check your daily Times Guide just to be sure.