It depends on the degree of the problem. Quarter horses have a tendency towards being slightly cow hocked and it doesn't usually affect them if it's mild. Slightly cow-hocked horses might not be suited to very demanding athletic sports, as they typically have more difficulty turning or stopping quickly, and uneven pressure on their hocks may give them problems with time. A severely cow hocked horse will have soundness issues and may be unrideable.
If it's a free filly and they thought it important to mention, it probably is a more serious case, ie. a companion horse/pasture pal rather than a riding horse.