Because the ball is rolling you may have some problems applying morph to do squash/stretch. See how it goes. If it doesn’t work out, another idea would be do the squash/stretch using material on the ball. That is, you could make a material to change the vertex position in world z to squash the ball into the ground.
Another material-based idea to consider is using distance fields to do a “soft-body” look for the ball, see here: Distance Field Soft Bodies - Rendering - Epic Developer Community Forums
Also, if “soft-body” is interesting approach to you, you could actually do a physics soft-body simulation for the ball and let the physics engine handle the deformation of the ball. Probably the best approach for that at the moment is Nvidia’s fork of UE4 called “flex”](https://github.com/NvPhysX/UnrealEngine/tree/FleX-4.19.2).
See here for some soft-body fun in UE4 + Flex:
