@Tarang - The current behavior of not including locked objects in a multi-select was most likely by choice at Miro or due to a technical challenge or constraint. Regardless of the reason, I agree with you that it could be improved. Apparently others do too as there is an existing Wish List Idea item requesting a multiple unlock feature - I suggest you add your Vote to this it here:

https://community.miro.com/wish-list-32/wish-list-multiple-unlock-1136