Translation:I'm bad at the piano.
Of course these expressions mean basically the same thing, but the original Japanese didn't have a verb. Depending on what points are being taught, it's the teacher's call as to how closely you must adhere to the structure of the original. I have had issues on this score with other languages.
And the opposite of 'heta' is 上手 ('jouzu'), for which the pronunciation (of 手 anyway) is also difficult to predict. A literal reading of the meaning of the characters, upper hand, might at least be more useful as a partial memory aid for the meaning than 下手, because even though we use 'to have the upper hand' to mean 'to have the advantage' in English, one often has the upper hand in something one is good at.