I think my father is so corny
What are some of the corny things he does?
He makes stupid puns, like saying that being corny is okay because corn is nutritious
I can see that you don't mean any harm, but it's still bad to call other people names.  Even though they can be annoying at times, family members just want to be loved, and we shouldn't tease them for being different.
They are corny, that's all.
While it may seem like it isn't an insult, your dad may feel like you calling him corny is hurtful. Apologize to him and only if he says that he doesn't mind being called corny, then you can continue to use the term for his silly jokes.