The iris, which is closely associated with the Boys' Day festival, does not have beautiful flowers.
Literal
This Boys'-Day-Festival [with-と] connection [gen-の] deep iris [topic-は], flower [topic-は] beautifully not-exist [polite-ません].
端午の節句 (たんごのせっく) is the traditional Boys' Day festival on May 5, now officially Children's Day. しょうぶ (菖蒲) is the sweet flag/iris plant, written entirely in hiragana here. The double は construction — しょうぶは...花は — sets up a topic-within-topic: 'as for the iris, as for its flowers (specifically), they are not beautiful.' This contrastive は on 花 implies other aspects of the plant may have merit.