It's pretty much impossible to rent a car without one! Even if it wasn't a rental car, it's virtually inconceivable that the OP would have a car without an Umweltplakette unless he had found it on a junk heap!

To the OP, just make sure you don't park it on the other side of the street, i.e. on the long internal road of the Messe. That's where the (mostly friendly and non-dangerous) hookers pick up a lot of trade. They're very obvious!