Can I use a Ku band satellite dish for Wi-Fi?
Bottom line is that it probably won't work very well if you need very high gain.

First things first. Satellite dishes usually have a receiver in them. You can't use the receiver. It is tuned for a different band. In theory, you could replace the receiver with a wifi transceiver, and then, if you aim the dish correctly, it could work.

But there is another problem. Most satellite dishes aren't all that big. Area of the dish matters a  lot, and most wifi dishes (that I have seen) are a bit bigger than satellite dishes.

As far as the general question goes, a parabolic dish will work equally well for all frequencies, provided that the dish diameter is larger than the wavelength you are trying to receive. But the RF gear at the focus must be the correct type for the frequency being transmitted.