When Bike Snob NYC poked fun at the Tweed Ride last week in his typically colourful manner, I was laughing my head off about the associations with colonialism and Civil War re-enactments: I never found Tweed Rides appealing for precisely those reasons. But I disagree with his conclusion that nice clothes on a bike are generally "not traffic calming." It's one thing to be dressed in what appears to be period costume, joined by dozens of others who have done the same while taking a joyride through the city. It's another thing entirely to be cycling to work in a suit and a wool overcoat, because that is what you normally wear to work.
In part, I think the idea of not wanting to harm Mary Poppins is valid - in the sense that a cyclist dressed "normally" looks more human to the driver. The way people process each other visually and emotionally is governed by a complicated system of simulation and self-recognition (this is actually my research specialty, so I've spent a lot of time thinking about it!). And from that point of view, it makes sense to speculate that the more "I am human! I am you!" signals we give off when cycling, the more empathy a driver will feel towards us. Dehumanisation, on the other hand, makes it easier to cause harm to another human being - because we fail to simulate their emotional state and relate to their suffering. And dehumanisation is facilitated by things like uniforms (one reason it is easier to kill soldiers and war prisoners, than civilians), or anything else that obscures individuality and hides signs of "humanness".
All of this is, of course, entirely speculative and none of it is to say that cyclists should strive to dress in any particular way in order to secure the driver's compassion. Only the certainty of criminal prosecution can reliably deter a driver from causing harm to a cyclist, and I hope to see that become the norm in the US in the near future. Until then, let's hope that drivers can find a way to see Mary Poppins in all of us, whether we are dressed in lycra, tweed, or something in between.