yes,i want to answer your question.Everybody,in fact,often says"not good"or "not very good" when facing the result of some things they have done."Not good" suggests the speaker is not satisfied with the result which maybe is adverse to him/her.Meanwhile,the mood of the speaker maybe is update.So we don't know if he/she will do it well henceforth.
Compared with the "not good","not very good"sugggests the speaker is full of the result,and maybe he/she will do his/her best to do it well next time.So i hope you can say"not very good"when we face a bad result,and take your courage to do it better.