Roses are red, violets are blue

Listen up, buttercup. This joke is like when you try to bake a cake but forget the sugar, right? It starts all sweet with the roses and violets, but then BAM! It hits you with a, "You're ugly and nobody likes you!" It's supposed to be harsh, like when your mom tells you to clean your room. The roses and violets part is a super common poem thing, then the author ruins it.