I don't recall seeing her at a character meal. You can usually catch her in MK back by where the submarines used to be. I think they call it Ariel's Grotto. She is there for pictures on most days, if I remember correctly.
You might also be able to find her at MGM as they have an attraction that features her but I can't remember if they have a place where you can meet her.
You can only see her at Ariel's Grotto in MK. This is because "flipping your fins you don't get to far, legs are required for jumping, dancing..." Oh you know the rest of the song. There is also the Little Mermaid show at MGM but you don't get to meet her.
We were there back in January when it was literally FREEZING outside and of course it was too cold for a mermaid to be out! So we found Ariel with Cinderella and Suzy Mouse in Toontown.