Apple is a sweet fruit where apple is a proper noun?

No, the noun 'Apple' is a common noun, a general Word for a type of fruit. It is capitalized in the example sentence because it is the first Word in the sentence.

A proper noun is the name of a specific person, place, or thing. A proper noun for the common noun 'Apple' is the name of someone or something specific, for example, the Golden Apple Inn in Glen, NH or Apple, Inc. in Cupertino, CA.