The problem with Fiona is that while she can be funny, she doesn't want to be the type of funny she's good at. This clip, for example, she just reacts deadpan to everyone doing bad French accents. That was a good bit and she works well as the straight man. But that's not what she wants to be. Much like how Jack works as the butt of the joke, but is too insecure to accept that role. Fiona wants to be the lead, the one who grabs the attention and makes everyone laugh. She wants to be a less angry Michael, a less stupid Gavin, a less drunk Geoff, and a more on topic Joel. But she sucks at that, and rather than try to get better, she attacked everyone who dared say she was bad at it.
Also, the fact that Trevor only hired her because he needed a black girl and consulted other people in the company instead of the AH guys she'd actually be working with.
https://youtube.com/watch?v=GiToPxuk6J4