Nimis here can have the equivalent of "really" in English: it smells really bad. If we are aiming at US English, "it smells too bad" is not as common. The base sense of 'nimis' (adv) is "excessively." In Late Latin the adj. nimius ("excessive" "too great") became a synonym for magnus, while nimium could be a synonym for valde even as early as Plautus.
that would be tam male olet or something like that (maybe the tam becomes something different in this case don't know i just had tam once in the sentence before this one, the same as spanish tan). nimis means too much.
EDIT: yes it's tam male olet. i just got that sentence 2 sentences after this one.