I have to say as well, I don't have the context of your child's extracurricular, but I struggle to think of one that isn't attractive to colleges if sold in the right way (especially if your child is genuinely passionate about it). Universities have become very suspicious (and rightfully so) of having a glut of extracurriculars (implying that the student isn't really serious about any of them) or of having 'paid for' extracurriculars (essentially those that only the wealthiest can afford).
Op here again. Thank you for all your responses, the different perspectives are quite helpful. DD has an extracurricular that she has been involved in for over a decade, but is a prohibitively expensive one, which is why I think it is unattractive for admission. The extracurricular is not a varsity sport at Amherst, but there is a club.
Original responder here. If it is something like dressage/polo/sailing I wouldn't lose any sleep over it. It can still be made into a really effective essay. I should clarify my original statement regarding 'paid for' activities: here I don't mean genuine extracurriculars that are prohibitively expensive, but rather ones that parents basically buy and the child doesn't work for at all. For a few years, there were a huge number of parents paying for their children to go to cultural or academic immersion camps that were meant to be a boon during admissions. It's not to say that you shouldn't send your children to these sorts of things as good ones can be hugely valuable, but admissions committees are aware that these (often eye-watteringly) expensive 1-2 week trips are solely a way to pad out applications. If it's an EC that a student is working on over a number of years, it's viewed quite differently.