Mudflaps'll make not a jot of difference to the covering that the ar5e ond of the
Ibiza receives.
It's all about aerodynamics.
The sharp edge of the spoiler detaches the airflow from the rear of the car. Behind the car a curling vortex of air pulls muck back onto the rear glass.
If the rear screen was more raked, this probably wouldn't be so much of a
problem. Or if Seat had put a slit in the spoiler (where it joins the window) to direct some of the air down the rear screen. But they didn't.