Is sweet is proper noun

No, the noun 'sweet' is a common noun as a general Word for any small piece of confectionery made with sugar (candy).

A proper noun is the name of a specific person, place, or thing; for example:

• Robert A. Sweet MD, Family Practice, Wichita, KS
• Sweet Water, AL 36782
• M&M candies
• "Sweet Georgia Brown", song by Ben Bernie, MaceoPinkard, and Kenneth Casey