She thinks that he has some kind of intellectual disability, and so she speaks to him very kindly and approvingly whenever he does anything (like in this case driving his own car).
Meanwhile, Kevin may be a bit slow to understand things, and is often portrayed as being goofy and clumsy, but I don't think he could be classified as disabled. There is a one-on-one scene with him telling the camera that Holly seems to be really into him, and "I am going to bang her".