- Mimi si mnene. - I am not fat.
- Wewe si mnene. - You are not fat.
- Yeye si mnene. - He/She is not fat.
- Sisi si wanene. - We are not fat.
- Ninyi si wanene. - You are not fat.
Wao si wanene. - They are not fat.
Mimi ni mnene na mfupi. - I am fat and short.
- Mbwa yake/wake[?] ni nene. - Her dog is fat.
it wouldn't take away from the meaning, but if we're doing strict translation, the given answer would be most correct, since '-nene' just means fat. Some adjectives have the tag 'person' on them because that's just how they translate, e.g. 'mwoga' translates to 'a timid person' and I can use it in a sentence as a standalone subject and it would make sense:
Mwoga amejificha (the timid person has hidden)
It's less common to do so with 'mnene' (and probably rude), so you would say 'mtu mnene'