vesicles
Colonel
When a native Chinese says "不错", he/she means they are satisfied with it and they like it.
For instance, when Chairman Mao and his followers were discussing his successor, they asked him what he thought of Hua Guofeng. He simply said "this youngster 不错". That was all it took for Hua to become the next Chairman of China. So apparently, "不错" is a stamp of approval for typical Chinese.
In this case, when the pilot said the J-20's low speed agility is 不错, he has given his stamp of approval to the J-20's performance at low speed. He is however over-joyed with its performance at supersonic speed.
For instance, when Chairman Mao and his followers were discussing his successor, they asked him what he thought of Hua Guofeng. He simply said "this youngster 不错". That was all it took for Hua to become the next Chairman of China. So apparently, "不错" is a stamp of approval for typical Chinese.
In this case, when the pilot said the J-20's low speed agility is 不错, he has given his stamp of approval to the J-20's performance at low speed. He is however over-joyed with its performance at supersonic speed.