Rocky Linux on Lightsail servers

Not at this time. Here is a list of the current OS Blueprints. https://docs.aws.amazon.com/lightsail/latest/userguide/compare-options-choose-lightsail-instance-image.html#compare-operating-systems

If you are looking for a RHEL clone such as Rocky, AlmaLinux would be a very similar offering that is command-to-command identical.

Hope this helps!