What would be your reaction or perception about a guy who is not neat, Smart & clean?
That's exactly the answer, as human being we tend to make perception of people within 1 min of meeting anyone. ( First impression is the best)
And your appearance does play a major role in that.
Having said that, an interviewer will never reject a deserving candidate just because he doesn't look good.
However if i am an interviewer and i have to select a guy out of the 2 guys ( both equally deserving) but one guy is disciplined, smartly dressed & neat compared to the other one. I will go with the first option.
So if you read my last sentence carefully, you would realize the guy got through to the final round based on his knowledge/competencies and not looks.