I don't think she's written abysmally. The writing just didn't get around showcasing her yet. There's been an awful lot of things going on in Delta so far, and frankly I'm not blaming them for not focusing on Mirage who was in no position to influence anything so far.
I keep believing that her character arc will start now that Delta squadron has to pull itself together to make up for the loss of Messer. Hayate has already said he'll work extra hard; Mirage will also have to pull her weight even more. One likely possibility is that she'll be either inspired by Hayate (and maybe Freyja) to overcome her insecurities, or maybe she'll get depressed and do the "wah poor girl has hit rock bottom" thing before being inspired by Hayate to overcome her insecurities... (you get the picture
)
She's a Jenius girl, no way they'll keep her playing second fiddle. (As I said before, if the show settles the triangle I'll be surprised if Mirage doesn't end up with Hayate. Pleasantly surprised, but surprised nonetheless.)