Off the top of my head, I would guess that, depending on the timing of the incident, this seriously inflames opinions against Charles. He flees London early in the year; a fire soon after that could be seen as his doing. Later in the year, well, it will cause a backlash against Charles, but it also weakens Parliament. At this point in the conflict, much of england is neutral; losing London like this is a setback for the Parlimentarian cause.