As is the case here. Cloud isn't immune to Ness' attacks because it's takes an extraordinarily durable mind to withstand attacks like those, and Cloud doesn't have that.

On top of that, Ness doesn't even have to attack his mind directly. He's got enough straightforward PK attacks to simply overwhelm Cloud in every way possible. The only advantage Cloud has is offensive. Ness has all of his bases covered.

Edit:

I didn't once say that Ness would just "send signals to his brain and fuck him up." Read my posts more thoroughly, Titus.