It is not Shinoda's fault. Kimura and Aoki are leagues away from the natural talents of Takamura or Itagaki, and are not as tough as Ippo. A trainer is a trainer, you can train a boxer with all your skills but it is up to that boxer to excel and utilize what you thought them, Aoki and Kimura (on his current weight class) are just not that very skilled. Beside Morikawa is using them as gag characters…it would be a little bit awkward if one of them gets left behind while the other gains a title, that is only my perspective though. XD
I've always sorta liked that part. I mean not everyone in the boxing gym can be geniuses or above average boxers. Aoki and Kimura to me feel more "human" in the sense that they are not superhuman boxers, but more or less boxers that do it solely because they love it and have to deal with the struggles of being an average boxer. Not to mention that those two make a great comedic duo.
In any case they do have talent, just not in boxing =P (Like Baseball =P). And in Aoki's case he actually gets laid!