I think its how you wear the kimono and what your body shape is like. I've seen some pics of westerners wearing kimono right and wrong- it looks terible when they wear them the wrong way like they are wearing a dressing gown or somthing.
Alot also depends on the kimono and obi's design and how the colors and patterns/pictures compliment you, i would like to wear kimono somday and i am a english girl but i know i could never look good in say, a pink or yellow kimono or anything else bold or brightly colored. Even if you are japanese you have to chose your kimono very carefully.
Padding helps alot as if you are realy curvy it helps neutralise your curves if you use and wear the right type of padding. Kimono are also supposed to be worn quite loosely apparently, if you wear them all tight it just doesn't look right wether you are japanese or not. If you have a rather angular face but have long hair, wearing your hair down helps draw attention away from places like your cheeks, nose, chin or forehead.
I also suppose things like plucking your eyebrows helps make them seem less pronounced and also wearing foundation will help make the contours of your face seem less obvious. Other touchs like using mascara on your eye lashs and eyebrow pencil on your eyebrows to darken them i guess would also help as i've noticed alot of japanese women have thick dark lash's and thin dark eyebrows.
Wearing the kimono correctly itself and having all the right other peices of clothing to go with it helps alot.
I think men find it easier to wear kimono and look alright than women in general. I think if you are a woman though, as long as you are slim, have reasonably long hair, don't have a broad nose or square face you should look good in kimono
.