As a person who teaches clinics on a regular basis, I will tell you what I appreciate in the people participating.
1. A spotlessly clean horse who is almost competition clean and pulled. No braids, please.
2. Spotlessly clean tack. Clean saddle pads. Clean wraps or boots.
3. Rider in clean and polished boots (half chaps if you must), clean breeches, clean polo, medical card and yes, for XC you must have a vest. A vest is just fine to wear in SJ and I encourage it.
To show up on a poorly groomed horse, dirty tack and poor clothing just shows disrespect and a sloppy approach to learning, IMO.