What is "bringing a pinwheel to a prizefight"

I don't think it's an established phrase, and I can't read the mind of the writer to know exactly what the pinwheel represents, but the general idea is clear: a pinwheel would be of little use in a fight, and may even get in the way. The writer also may have have intended a sense that the pinwheel's normal purpose is inappropriate: a pinwheel's purpose is to look pretty and make people feel good, but a prizefight is an adversarial event where the purpose is to beat the other person. Similarly, talking about healing the nation makes people feel good, but there's a worry that the campaign should be more adversarial.