Extremely important but there is also attraction, the girl's performance style, and whether or not she is actually good, which means that I won't always get the card because of the costume but will be tempted. Whenever I read "barefoot" or "thigh high boots" it usually increases the possibility of me getting the card. More to the point, I like Goth outfits, cop and military costumes, but also sweet and elegance outfits like Jenna Sweet's "Granny Smith" card or Marta's "Neapolitan Coffee" card.
Some people do not care about the costumes AT ALL and they get upset because the girl might keep it on too long or might not take it all off. For me, I love the costumes but would like it if the dancing fitted the costumes. For instance, Viki "Diablo" card would have been better if her dancing was more tribal and dark. A lot of the times the routine is the same regardless of the outfit, which makes the change of costumes pointlessly.