I think our difference lies in definition of good.
I see your definition of good as being a translation of statistical averages (better/worse) to judgementally good (better/worse) based on an arbitrary line somewhere around average (or as Level says "somewhere around 85%"). The problem I have with that is the stat is not judged in a vacuum.
The isolated stat allows you to organize from better to worse. But, it doesn't define "good" until you draw a line and say everything to this side is good, and everything to that side is not good. Based on the number being mentioned, that definition of good is contrary to the practical application of what I see on teams (teams not in a fantasy league). Because there are too many "not good" contact hitters hitting in the "good" contact slots.
I accept saying a player is above average, average, or below average statistically speaking. That is a simple statistical determination. But, good is in the eye of the beholder.
I define good as being within the acceptance and tolerance levels of those who are in a position to judge (coaches who make out the lineup card and scouts/GMs/teams that draft players). Because, if the player is judged as "not good", he moves down the lineup and eventually to the bench.
"Good" is subjective, and dependent on other factors. To me, if he is still in the early part of the lineup, to those who make the decisions, he is good. Now, if production enters into that decision, then that's real world application, and why I said it may vary by type of hitter, and maybe even team philosphy. The more production, the more the tolerance. His K-ratio is acceptable for what he brings to the table...therefore, it is good.
So, "good" can be worse-than-average because it is within acceptable limits as defined by those who make out the lineup card and/or draft players. "Not-good" ends up not playing. Isn't that the ultimate deciding factor?
I took the question to mean what is good, meaning good to stay in the lineup (in other words, if the asker is concerned that his son's K-ratio is too high and may ultimately get him moved down or out of the lineup, then the acceptable level may vary based on other criteria).
Using my son's team as an example. The leadoff hitter (.310/.408/.871) also had a K-ratio of 21.6%. Your definition says he is not a good contact hitter because he was worse-than-average (isolating the stat). Yet, he remained the leadoff hitter on a top-40 D1 team, and was drafted as a junior. So, he was within the acceptable/tolerable limits of those who judge. And, ultimately, that makes it good/acceptable.
So, when the question was asked "What is a good K/AB ratio?". I guess the next question should have been "As determined by who and for the purpose of what?". That may have saved a lot of bandwidth.