She's one-dimensional, has minimal personality, and doesn't grow as a character (or at least not very well).

She changed at the end, but if you think about it she hardly developed. Yes, she was strong during Let it Go but then she went back to being weak.

Basically, she had very little development, and a sparse personality. And her yelling "it's love" at the end of the movie, thanks to her sister's input, really failed to convince me that she had grown as a person.

Well,thank you very much for breaking my heart.

Could've been a lot better if she had been the main focus of the movie.

Well, she learns to accept herself. It's just not executed/written very well.

^^^ What Mongoose09 said.

They could've done more/better, but I don't see at all how she's 'flat'.

I don't see how she's flat, or at least i don't understand your criteria for deciding how well-developed a character is. Like if you're counting poca's stopping a war toward her development, why doesn't Elsa saving Anna's life and bringing back summer count toward anything for her?