They changed a lot here from the novels. In the novels her evaluation was given prior to graduation as a knight, and was low because she could not control a knight well, not because of a screw up during a mission (students don't get any missions that quickly).
Emma wasn't able to pilot the mass produced models well because they have an assist feature to help regular people pilot them and she was spending more time fighting the assistance than controlling her knight. Liam's knight Avid was notable in that it lacked such assist features as an older model, and was the primary reason Yasuo (Liam's sword instructor) had Liam use it, so that he would struggle to use it properly and buy time for Yasuo to slack off or escape. Emma practiced on a broken simulator throughout her childhood, and the simulator was considered broken because it didn't have such assist features, and managed to master piloting without one (I think the presence of that broken simulator was also because of Liam, he left it out and forgot about it or something). Liam later coincidentally sees a video of her struggling and immediately identifies the problem that she's fighting the assist, and she eventually gets a custom knight without it.
It's be like aim assist in fps games constantly dragging you to a point on enemies while a pro who played without it is trying to flickshot heads and getting their aim screwed up by the aim assist, and getting confused why they're missing so much, and it not getting better even after messing with sensitivity settings.