The phrase "sweet peanut butter" is an oxymoron, as it combines two contradictory words. In this context, "sweet" means pleasing or agreeable to one's taste while "peanut butter" typically has a strong and distinctive flavor that may not be considered sweet by most people. Therefore, the phrase is likely used in a figurative sense to describe something that is enjoyable or delightful, despite having an unexpected or unconventional nature.