Omollo
So far you have only given two reasons:
1. Support for FGM
2. Being silent on EJK and support of tribal chieftain.
On the first, Ngugi comes from a tribe that practiced FGM, his stories are centered on that because that is how hhe was brought up. Same thing can be said of many other ridiculous customs being practiced all over Africa. He was also a fictional writer so his stories arent necessarily a representation of reality.
On the second point, he is entitled to support whomever he chooses as president. How many other renowned writers, poets, have been known to go against the volatile political landscapes of their times. Ngugi has paid greatly for some of his actions, I think trying to simp[lify his works to only these two issues would be too simplistic. I can't claim to have read all his work, but I have read some and he writes well.