All the other tags that fall under the Amazon Web Services umbrella follow the convention of starting with 'amazon-', this appears to be the only remaining exception.
As per Amazon's description:
Amazon Route 53 is a highly available and scalable Domain Name System (DNS) web service.
One possible argument for leaving the tag would be that Route 53 is viable as a stand alone service (unlike something like amazon-ebs which cannot be used without amazon-ec2). In this case, I would suggest that amazon-route53 should at least be a synonym.