~6 YPA is very bad for college football. I think ~8 YPA is average. It isn't the only stat to determine how good or bad a QB is, but it reflects completion % and success moving the ball down the field. Here is Ewers after OU.
You can see statistically Ewers didn't have a good game until Baylor. I would say they were all bad with TCU and Kansas being garbage. Maybe it was understandable against ISU's defense. Against KSU he did well in the 1st half so it wasn't the defense. It was on him and Sark not adjusting to KSU. It looks like Sark figured something out after Kansas.
Against Baylor, he didn't have many attempts but did good when called upon. The problem was giving up 8 points. Obviously concerning, but these types of things should get better as he gains experience. He wasn't bad with interceptions which was a concern coming into the year. That is on the positive side. I would say overall his play was a disappointment but understandable with his inexperience and seems like Sark is still trying to figure out how to use him best.
ISU - 6.6
OSU - 6.5
KSU - 6.4
TCU - 4.4
KU - 5.1
BU - 12.1