[It's a very informal way to address... well, anyone. But Balthus is not exactly wrong and doesn't appear to mean insult, so he doesn't correct anything.]
You do not appear to be someone who does much gardening.
[That, in turn, is not meant to be insulting, but just a blunt observation. Balthus praises himself for fighting, and if he has any other skills then Dedue truthfully does not know about it. He isn't a poor student to the Professor in the least, but it's different than how Sylvain pretends to be an idiot.]
no subject
You do not appear to be someone who does much gardening.
[That, in turn, is not meant to be insulting, but just a blunt observation. Balthus praises himself for fighting, and if he has any other skills then Dedue truthfully does not know about it. He isn't a poor student to the Professor in the least, but it's different than how Sylvain pretends to be an idiot.]
What are you planting?