I think not. Typically, oxymorons have an adjective which describes a noun, and they are apparently opposites, such as jumbo shrimp. The word shrimp implies small, and the word jumbo is a descriptor,...
I'd say it is. Repetition is something (an idea, a word, a phrase etc. etc.) which is REPEATED for effect. Here, the word 'sweet' is repeated. It is done for effect. Hank Williams in one of his songs...
Sweet potatoes are not actually sweet. They are called sweet potatoes because they are not bland, and people back in the day had never tasted such a good potato before. Because they considered candy...