"That meat smells bad."
Translation:Cette viande sent mauvais.
No, neither in French nor in English. If the sentence were "The meat is bad", then "bad" would be an adjective. But here we have "smells bad", where "bad" is an adverb describing how the meat smells.
(Do not be mislead by the fact that the English word is "bad" rather than "badly". Not all English adverbs end in -ly).
To put the matter another way, you cannot always decide a priori that a certain word by itself is an "adverb" or an "adjective". You must look to see how the word (or phrase) functions in the sentence.
I mean that "bon" or "mauvais" are usually adjectives, to be used with nouns, like: un bon sandwich, une mauvaise nouvelle
In this sentence, with verb "sentir", they modify a verb and are used as adverbs, i.e in an invariable form.
This is a special construction that does not apply usual rules.
Let's clarify what "that" stands for and what "ça" translates.
"that" can have the role of an adjective, ie, accompanied by a noun : "I see that flower", which translates in "je vois cette fleur" (singular feminine of "ce")
"that" can have the role of a pronoun, ie, standing alone: "I see that", which translates in "je vois cela" or "je vois ça".