I think it's because she's got very short hair and I've never imagined a mermaid with short hair before xD :P
But I dunno, I'm not sure..

She'd look great as one but I honestly think she'd just appreciate mermaids up close rather than actually turning into one - she didn't go to London because her people needed her so if she was a mermaid she wouldn't be much help

I can see other girls becoming mermaids: Tiana would make a great mermaid and open a resturant under the sea although obviously fish wouldn't be in the menu:) Cinderella would do pretty well too, I can see Rapunzel and Aurora being great as mermaids, singing and exploring all day long, but I cannot picture Snow White as one. Sharks and other dangers are way too over the top for her also not sure she would like such an utter change of everything in her life and in her look.She would be scared and maybe even sad about it.

It doesn't seem like her, I don't know... Jasmine I think wouldn't mind, and Aurora'd be perfect considering mermaids are known as being beautiful, and even a bit charming and seductive.

There would be the least opportunity in her world I imagine...

Easy..Snow White