"License plates are the property of the state and must be visible, unobscured, and attached only to the vehicle to which it is registered."

If the issue is a person removing a piece of tape covering a license plate, then really nothing happened. The citizen unobscured state property which is supposed to be visible. It would be like removing a sticker from a speed limit sign, or picking up litter.

If it's some other piece of plastic attached to the car itself, well IMO that's what courts are for. That cyclist goes before a judge and maybe some technical reading of the law interprets what he did as tampering with the vehicle. Another completely reasonable interpretation though is that the cyclist was making the shared roadway that the car was on safer for everyone else.

The police weren't justified in charging him for criminal mischief and I bet you it will get dismissed, and he'll win on a false arrest claim as well. Those should be starting points for your analysis.