You can legally put a note on someone's car as long as you leave a respectful and not threatening note and cause no damage to the vehicle.
Putting a note on someone's care is legal like for parking issues, parking disputes, or if you crashed into the vehicle and nobody was around to report it to.
Although if placing the note on the vehicle causes damages, constitutes harassment or involves trespassing on private property, then placing the note on someone's car can be illegal.
But simply placing a regular note on a person's car is not illegal in itself.
When placing a note on someone's vehicle you should avoid damage and avoid touching the car like lifting the wiper places, which can lead to allegations of vandalism if a scratch or some other damage occurs.
And never place any notes on someone's car that contains any threats, abuse language or racist languages as it can lead to criminal charges.
Leaving notes on private property, restricted property etc can also be considered trespassing.
And for disputes it's also best to keep the notes respectful and brief and focused solely on the issue like for example, Please don't block this driveway".
As long as you don't threaten someone with the note then it's okay to place the note on someone's car.
I've placed notes on people's cars and vehicles before and it has never been a problem.
Just don't threaten or do any damage and you will be fine.