Dressing in an attractive manner certainly helps, although there are many singles who only want to date other singles who "like me for who I am."
Regardless of attire, good grooming is a must.
Sexy is an advantage unless it is taken beyond the realm of good taste. Good taste is a personal opinion, of course.
Speed dating is very visual. That can be an advantage or a disadvantage.
If you are very attractive, that is a great advantage. Many people will choose you unless you come across as very vain.
If you are average in appearance, it is not an advantage or disadvantage. Some people will choose you and if you choose them, also, you will probably get some matches. If you only choose the most attractive people in the room, you may not get matches.
If you have several unattractive features and don’t compensate for them, speed dating can be a disadvantage.