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'