I'm pretty sure my teacher is incompetent.
Why do you think they are incompetent?
The questions he asks are too difficult, and the answers he gives don't make any sense, so I'm not learning anything.
You shouldn't insult a person of authority even if you don't agree with them. You should respect people more.
I don't know why you think it is not acceptable for someone to insult teachers.
It's possible that your teacher is actually quite good, but you just don't understand. Either way, insulting him isn't going to help you learn. If you're having trouble you should ask him for extra help.