In what discpline of riding is it actually acceptable to do so??
Trail riding. Mounted games. Campdrafting. Endurance.
I HATE open shows. I detest that people/horses are judged on appearance as much as riding ability. I hate that it panders to the rich, and those who don't train their own horses but pay others to do it for them. I still show occasionally, but I do it on my terms. I recently rode at our zone showriding competition in my ASH get up. Got a few odd looks but still placed well. It should be about how you ride, not about how you look.
That's the reason I gravitate toward things like MG and sporting (only judged by a stopwatch) Campdrafting (judged on how you ride and work your cow) Polocrosse (the score) etc. Clean cut.
The only showing I am interested in is ASH shows, because they have reasonable expectations. Simple dress code, and they mention in the rulebook that NEW and EXPENSIVE gear often looks tacky and older, well cared for gear looks better.