Is this chassis to counter the new frigates that are coming out of JPN?
Military equipments are rarely designed to "counter" anything, especially unlikely in a force as reliant on consistent fighting doctrine and force structure as the navy. Type 054Bs most likely will fill in the role 054A always filled (which is mainly fleet ASW and some degree of AAW, assisting the destroyers), only of course substantially more capable.
While the Mogami class is designed to replace old DEs and serve as escort ships in the mine-hunting fleet of JMSDF's new force structure; while also being capable enough in ASW (first in JMSDF to carry VDS), ASuW (8*17SSM), and AAW (03 Chu-SAM kai) to fill in in the main fighting fleets.
Type 054B is a relatively specialized "blue-water" warship meant to complete its assigned tasks the best it can; while the Mogami class is a multi-purpose warship that focuses much on littoral warfare, hence the significant emphasis on mine-hunting unmanned vehicle operations. These two are hardly comparable in their respective roles to be considered "counter" to each other.