I wouldn't say that she's overrated, I understand where people are coming from with liking her. I can't say I was impressed though, but I give her the benefit of the doubt of being in BBS, a game I found to be overall lacking as far as characters were concerned (the best characters imo were Terra for concept and ironically Eraqus, who was about the most minor character aside from Vanitas).
What irks me the most is that the game doesn't even try to paint her as the saint most of her fans do. The Fairy Godmother, Ven and Yen Sid all tell her what she's doing isn't quite the way to go and is even the wrong way to go about things. Way I see it she's a big part of why Terra fell that badly into the Darkness and I can't say she made all the right decisions, like leaving Ven alone in Radiant Garden after she just saw that a violent maniac was out to get him and Terra.
Her character's also kind of contradictory at times, from believing Terra's in the right way to suddenly believing he fell to Darkness because the same old woman she told Ven not to listen to yapped her mouth ONCE. Also the issue of thinking Ven might finally be ready to spread his wings but, durr, no. Guess not. Circumstances do explain some of it but the overall stringing of her behavior is just poorly done, imo.
I might be able to appreciate her depending on her role and development in future installments, and I do appreciate her concept of being the caring, considerate friend who's just too zealous so she helped ruin everything. At the moment though she's a mediocre character until she gets some time in a game whose character structure doesn't make me lose by raging.
It's just an excuse to milk Aqua and the series.
I'd sooner say it was an excuse to milk the series, less so Aqua herself. As said before she
is the only member of TAV available to do anything with unless you want to play as Terra's Lingering Sentiment. Oh no, I know, let's play as Ven. Achievement - go through 20 hours of him sitting in the CoW without skip-scening.
The Realm of Darkness is a place whose story Nomura wanted to tell for ages. I wouldn't necesarilly go as far as say there're other scenarios, but they can use the chance to show us what Mickey's been doing there during KH1 and maybe tidbits about Riku and other people who might've stumbled in there.
Eternal kudos for showing us the Apprentices become Nobodies.
tl;dr it's a story that needs to be told, and with Nomura saying that Aqua's the true protagonsit of the BBS line, it's about time he lived up to it rather than have her be an onlooker.