Sure. A 128GB iPhone would be useful to fit more music on and photos. The same reason I want to update my 2012 iPad (to have a bigger storage). The mini would be a fit in between?
My experience with the mini is that it can be less stable - i.e. it can drop the signal - re wifi than is the standard iPad; however, it is supremely portable. Only you can answer the question of what it is that you want (or need) from such a device.