They might. They might check in with her, telling her that a report was filed, and that might be enough to get her to think twice next time. Or maybe she already has a record, and this adds to it.
I’m not a huge fan of calling the police over small things–but this is out of control enough that she could be in danger of escalating to more harmful behavior. Getting her to take a gut check isn’t the worst thing in the world.
In many jurisdictions (e.g. California) its a crime in its own right to.throw an object at a car:
IANAL but it seems like it would be assault in jurisdictions where it’s not.
Of course in practice convincing the cops to lift a finger is going to be challenging but if they are motivated to do so I think they would find something to charge the milkshake thrower with
Assault with a congealed weapon? 
That’s an old Gracie Allen joke. Although it was about feeding elephants rather than littering.
My dad used to say that only two things could legally come from a vehicle: water and chicken feathers.
The reaction of your local police depends yugely on where you live.
A big city will yield zero response. A rural hinterland will probably yield zero response. A hyperactive overendowed underworked department covering a toney suburb of Important People? They’ll be all over that wacko woman.
No evidence = nothing done.
If the crazy driver was waving a gun, then even without a corroborating witness a report should be filed.
The OP had an unpleasant encounter, but I’d let it go.