I told my neighbor I found his poetry to be amateurish.
Did he ask you to read his poetry?
I found it on his table.
It was impolite for you to tell your neighbor that his poetry was amateurish because it isn't helpful. Criticism is inappropriate if you can't offer any help to go with it. I think you should apologize to your neighbor.
You mother's hamster hair is amateurish.
Telling someone they are bad at something is unkind, especially when they didn't ask for your opinion in the first place. How would you feel if someone did something like that to you?