Is it necessary to use only flexible cable for battery termination in a UPS or can we also use hard cable for the same purpose.

It is easy to terminate as compared to hard cables.

Flexible cable is good for movable equipments like lifts, robots and etc.

It's only necessary if you want ease of installation/termination, ease of individual cell/battery removal, freedom from fear of cracked post seals/battery cases due to mechanical stress/thermal expansion, basically all the advantages of using flexible cable. The main downside is if your termination practices are poor you may experience rapid corrosion between the fine strands and the connector.