This is so out of line.
Given that you presumably didn't include or fully complete the form, if anyone was going to throw the term "mentally limited" around, then, you know... except I'm not as rude as that and I know mistakes are easily made in completing these applications when you've got umpteen different forms and a stack of paper on top of it. We're all human. (And so are they.)
I'm just saying. There's nothing wrong with expressing your frustration, and I'd be steaming mad too in your position, but maybe you could lay off with the namecalling. There's no need for it.