I am sure that most of you have heard the expression "you can't judge a book by its cover". I would suggest that isn't entirely true! Perhaps you a cannot judge a persons character, values, or intellect by their outward appearance, but there IS a lot of information you can assume from outward appearances.
For instance, how a person grooms themselves can tell you a lot about them. Does he/she need a haircut? Untrimmed nails? Facial hair need trimming? If the answer to any of these is "yes", then you can generally assess that the person does not spend much time on detail, is uncomfortable or inexperienced in formal settings, is not concerned with other peoples opinions, and making a positive impression is not at the top of their priority list.
How are they dressed? "Loud" colors? Can come off as being over confident or difficult to work with. Clothing not pressed or looks like they spent two minutes getting dressed? This will generally say the person doesn't usually put forth their best effort or produces mediocre work. Too much makeup, cologne/perfume, or jewelry? Can be considered a person who has questionable priorities and decison-making ability.
These are just a few examples of how others perceive your appearance. Does this mean that everyone who makes one of these mistakes is doomed in the hiring process? No, but be aware that you have an uphill battle if your first impression is not top notch. When you arrive for a job interview you should have put 100% into the process. Making a favorable first impression is critical.