Is the wen composed of living tissue complete with nerve endings, or is it just a mass of dead tissue like hair, nails or antlers in animals? If the latter, then wen "surgery" would be no more traumatic...at least to the fish...than a haircut or manicure for a human.
But if it's living flesh...it's mind-boggling to think that people breed these animals specifically for their ability to grow these weird structures on their heads, and then start carving on them when they grow! Wouldn't this require anaesthesia, followed by a lengthy and slow...and painful...healing process?
I'm a bit traumatized when I trim my dog's nails if I happen to cut a bit too close and hit the quick, causing a sharp pain to the animal and producing bleeding. Don't even want to think about unnecessary cosmetic surgery.