The only reason Aeris seems overated is because she has a major part to the game. I mean, nobody says that Cloud's overated because he's the main character, but when you say that a sub-character is overated it's because sub-characters usually have a smaller part in the game from a main character. Aeris is different.

When Sephiroth summons the meteor what stops it? HOLY! And without Aeris summoning Holy what would be there to stop the meteor from destroying the planet? Because of Aeris' tagalong attitude she saves the day the moment in the game where she says 'I want to come along!' You don't see Cloud summoning Holy now do ya?

Even though she does get annoying at times with her goody two-shoe ways (hee hee...I always wanted to say that. ) and her tagging along with Cloud (not to mention flirting with him in front of Tifa) means that she sees Zack, since Cloud took the image of him, after the Nibelihiem (don't know if i spelled that right) incident. That's the main reason why she likes Cloud so much.

So not exactly overated...she just had a big part to play in the game.