ADHD is not part of normal variation and it's not based in personality. It's a condition chiefly caused by distinct biological differences in dopamine transporter function. ADHD's effects are in executive function: memory, attention, inhibition. Granted there are some personality traits associated with it, but those are secondary. That said, personality and presence of other conditions likely has some effect on who is diagnosed / misdiagnosed.

As for whether ADHD is disordered, there is evidence that it was a positive adaption in more primordial settings. Greater distractability may have equated to faster response to predators or more success in hunting. However, to be a disorder, a condition simply needs to be presently maladaptive. In modern society, ADHD typically is.