Yes but this only applies when the adjective is attributive (before a noun). When the adjective comes after the verb it is not declined. In this position it is either a predicative adjective if it comes after any conjugated form of the verbs sein, bleiben, werden (e.g. Die Zitrone ist sauer) OR if it comes after any other verb it is technically an adverb qualifying the verb and is also not declined. So in this case "sauer" is actually an adverb qualifying the verb "schmeckt" (how it tastes) and does not change with gender or case of any noun in the sentence.
In our country we have a fruit that we call it limo shirin literally meaning sweat lemon and it does tastes sweat without a single trace of sour taste, but it should be eaten fast after it is cut or it will turn bitter in a few minutes. Then I checked the dictionary and in english it's called Chino.
When the adjective is placed after the verb, it uses its "normal form" (I don't know the actual name), which is sauer, and when it's before the noun, describing it, it should be declined: Die saure Zitrone (The sour lemon). Der saurer ... Das saures ... Die saure ...
Hope it helps!