Mokuton is not yang release. Most likely it reacts to yang release because the yang release stimulates growth. If yang release truly is just physical energy then this could be interpreted as providing physical energy to the plant stimulating growth. It's not so much that mixing yin and yang is automatically better, but that what you can do with chakra changes with the composition of yin and yang. That particular explanation would explain Naruto having an easier time learning KB than Bunshin. Naruto, who is naturally very yang heavy to begin with, has the yang portion of Kyuubi sealed within him. Bunshin is not yin release, but one expects that it is a very yin oriented technique. Naruto most likely did not have the skill to achieve the correct balance of yin and yang at that time. Kage Bunshin, however, I would expect to be at least 50/50 yin/yang, or perhaps more yang oriented. Yes it splits the consciousness, but there's no interaction between the consciousnesses (really? that's a word?) of the clones. The creation of a physical body seems to be the bigger deal. The result is that Naruto has an easier time learning this much more advanced technique due to it being more up his alley. That explanation is so much more satisfying to me than the bullshit fanon "he has too much chakra to do a Bunshin." One of the most annoying cliches out there and it's totally unnecessary. People just want to be able to paint Naruto as being a hidden super talent, which ends up trivializing his struggles.