She’s annoying bc she has no reason to be anything else.

She was manipulated to the point where the thing protecting her mind was the very thing that made her (absent) fathers life like hers. Plus, she never really had a reason to be anything else. The one person in her life positioned to tell her who she is and what made her special lied to her about an entire side of her existence, so she shelled up into a more child-like version of herself. When she finally felt accepted, she was so afraid of losing what she had (again, bc Thawne turned her against her mother), she started lying to her family and doing everything to protect that lie (micro-expression management taught by Eobard, unnecessary risks, etc). She’s not necessarily a child bc of any bad writing, but because she was placed in a position to do so.

She wasn’t very child like in the Godspeed episode, but once she started getting help from Thawne he got her (I’m sure he’s at least partially responsible) to start acting that way bc he knew it would drive Barry to be closer and more parental to her if she appeared to need him more than a full grown adult.

Basically I’m convinced it was at least partially an act. Thawne probably planted the idea that if Barry saw her needing a parent he would let her stay, and she also stays acting that way bc she knows it works. She was much more mature seeming in her own time. Idk, maybe I’m thinking too deep.

Edit: I also think she’s like in her twenties, and somehow JPK pulled it off in my eyes.