I’m still bad at the guitar, but I practice every night.
The person, who is not good at imitating Americans, is good at imitating French people.
I’m bad at brushing my teeth.
Think of someone holding out their hand at shoulder level. Anything above that equals "skillful." Everything below that equals "unskillful. So, in the case of this word, "below hand" means "unskillful."
If you're bad at something and you complain about it, then you're just a hater (へた). Don't be a hater.