The Hungarian workers are very slim - this was marked as wrong - surely thin (referring to people) is slim - by definition - slimness being the more appropriate English adjective relating to a thin person. Alternatives also include - lean, lithe, skinny and svelte which are probably a bit more specific. Does the dictionary need expanding?
It's slightly positive, if anything. Looking at the negated versions: "nem karcsú" is a rather straightforward negative judgement, "nem sovány" can be both positive or euphemistic/sarcastic, "nem vékony" cannot really be positive but it can still be neutral or euphemistic. It doesn't sound like a huge judgement.