Is it hard for a firewall, such as Cisco ASA, to be an NTP Server?

The direct answer is, NO, it’s not hard at all. It’s just a matter of supporting the NTP Server capability on any firewall appliance’s software.

The right question should be: why to implement such critical service on a device that is designed, beforehand, to protect services?! It’s just the logic! NTP opens vulnerabilities, and must not be implemented on firewalls.

Hence, you cannot find a way to configure a Cisco firewall (or any other) as a NTP server!